Let's examine this question analytically, starting with the question of returning characters, which are surely most likely to take the majority of remaining spots, regardless of the ultimate number of seasons. I can see three obvious primary factors that would argue for or against any of the "usual suspect" candidates, each of which has been noted previously in this thread:
1) The character's place in the Soul Calibur chronology and narrative continuity. This is probably likely to be the single most determinative factor, given the manner in which this game has been positioned as a soft reboot. It's true that, through both direct interviews and what we have now experienced of the game, the creators have made it clear that they are willing to retcon certain aspects of the story--and to be perfectly honest, as someone who has played the games of this franchise for more than two decades, I would say consistency of narrative between entries has never exactly been strict. That said, it's pretty clear that the devs have (in their character selections and plot elements presented so far) focused heavily on the earlier story arcs. Characters introduced in the earlier games (soul Edge and Soul Calibur I, II, and III) are probably much more likely to be represented than are those from later games (SCIV/BD and SCV).
Notably, there are no characters introduced in SCI or SCIII which are not yet in SCVI, with the exception of IIIs "bonus style" characters. I think we can probably all agree that the SCIII bonus characters are unlikely candidates, with the exception of the three characters who were made full styles in SCIII: Arcade Edition (Hwang, Li Long, and Amy) each of whom also featured as a "regular" character in either a previous or later game in the franchise (and each of whom existed in the lore before SCIII), unlike the other SCIII bonus characters. Add to this list Cassandra, Lizardman, Rock, Setsuka, and Yun-Seong, and we have a list of all main roster characters from Soul Edge through Soul Calibur III who have not yet been rostered--provided we exclude mimic characters, non-playable bosses, and Seong Han-myeong. Well, technically there is also Necrid, but the licensing issues make him an unlikely pick, even aside from the fact that he was never particularly well-embraced by SC fans.
Characters introduced in Soul Calibur IV are clearly a possibility as well, per devs leaving that door open in interviews, but I think we can agree that these characters are at least substantially less likely. With the exception of the four SC IV/Broken Destiny guest characters, and the various bonus characters of that game (who, unlike the SCIII bonus characters didn't even have unique fighting styles but were pure re-skins of established characters already present themselves in that game), there were only three characters introduced in SCIV and Broken Destiny: Algol, Dampierre, and Hilde. It is probably highly unlikely that we will be seeing any of the SCV edgelord (pun totally intentional) additions and, personally, I am quite happy with that state of affairs. For the record though, those characters (again, excepting mimics) would be: Aeon (as a distinct style from the earlier Lizardman moveset), Leixia, Natsu, Patrokolos (and the Alpha Patrokolos style variant), Pyrrha (and Pyrrha Omega), Xiba, and ZWEI.
Previous guest characters have never returned before in a Sou Calibur game, but given the emphasis the devs are placing on cross promotion of guest characters and the recent retcon of God of War, I wouldn't entirely rule out the possibility of seeing an updated Kratos, particularly as he has never featured in a full console version anyway. The other eight guest characters/styles (Heihachi, Link, Spawn, the Apprentice, Darth Vader, Yoda, Ezio Auditore, and Devil Jin) are highly improbable repeats, but you have to wonder if Kazuya isn't a possibility for a new Tekken guest, if only to fill out the three generations of Mishima fathers/sons.
2) Uniqueness of the fighting style. Characters who do not fit into a niche that is well represented by another character already in SCVI are probably more likely to be included as DLC than are those who have a similar fighting style/moveset to such characters who are already in the roster. It is obviously difficult to qualify how "similar" characters are in this respect, beyond generalizations. But, for example, I think we can all agree that Hwang and Yun-Seong would be unlikely to feature in the same season pass, given their strong similarities in both presentation and fighting form, whereas someone like Hilde, who does not have a strong analog, would probably get a leg up in her chances from this consideration.
3) Popularity. I honestly think this is likely to be the weakest of the "big three" factors, by far. Obviously Namco/Team Soul had an objective with conducting their popularity poll, but I suspect it had more to do with maintaining hype/preparing promotion than with being certain that they picked the most popular characters. For one, it will not have escaped the notice of the development team that the numbers who participated in that poll are infinitesimally small compared to the market share ambitions for any game that is going to stand a remote chance of winning back its production cost, nevermind being highly profitable. For another, I just think the devs have bigger concerns (balance, licensing, even their own personal impressions about the best fit) that are likely to override the populairty poll and other forms of social media feedback in general. That said, I wouldn't toss this factor out entirely: there's a big difference between, say Setsuka popularity and Danpierre's
Lastly, there are any number of additional factors which may make the case stronger or weaker for particular characters: whether said character features prominently in the single player narratives, whether there are well developed art assets already represented in the game, or (in Lizardman/Aeon's case) the fact that the style has already been partially worked out.
Keeping all of these factors in mind, let's look at the case for what I believe are the top nine contenders, and then some leftovers, roughly in descending order of likelihood:
Amy:
Continuity (strong): Amy has featured more prominently than any other as yet unplayable character across the two story modes of SCVI. Her role as a background character was also well established in SCII and she was introduced as a playable character in SCIII/SCIII:AE, so she falls very much within the timeframe of events covered by SCVI, if obviously more towards the tail end.
Uniqueness (moderate): Amy is obviously superficially similar to Raphael in terms of fighting style, but anyone who has mained either of them will tell you that they actually play remarkably differently. In any event, there is only one rapier-wielder/western duelist character in VI so far, and two would not be altogether unexpected.
Popularity (strong): Amy remains an exceptionally popular character whose absence was possibly the single biggest complaint about SCV's roster after Talim (regardless of whether or not Amy and Viola are, as a narrative matter, the same person or not; incidentally, I hate to burst the bubble of anyone hoping for Viola over Amy, but I'd say that's a virtual impossibility, per numerous of the factors explored above).
Hwang:
Continuity (strong): Hwang is a SB/SE character and thus one of the earliest characters of the franchise not yet included in SCVI. He is featured/referenced in both of SCVI's story modes. In general, he has that "classic faces of the series" vibe that is emphasized in VI. Notably, he also has a habit of edging his way into games through the back door: he is reskinned as "assassin" in SCII and he is a bonus character in SCIII.
Uniqueness (strong): This analysis would of course change if Yun-Seong were included first, but as is, there are no dao wielders in SCVI thus far. At one point Xiangua's style was quite similar to Hwang's, but the two styles diverged a long time ago and are quite distinct at this point (taking Hwang as he last appeared in SCIII:AE).
Popularity (moderate): If there's any reason Hwang may lose out to Yun-seong, it's this: Yun-seong, although a mid-tier character at best, has always had a loyal fan base, whereas Hwang (whom I personally prefer of the two) is rarely bemoaned when he is absent from a game. Regardless, the dao characters have been absent for a while and I judge one of them virtually certain to appear in the first (hopefully not only) season pass, and I give Hwang the edge.
Lizardman/Aeon:
Continuity (strong): Lizardman (or lizardmen, or Aeon, or whatever permutation) has been present in some form in every game in the franchise since Soul Calibur I. His analogs are present in the story modes of VI already, and there is already at least a partial build of a moveset for that purpose. I suspect that he has been planned for inclusion from the start but that the development team have struggled with discovering the ideal way to introduce him.
Uniqueness (moderate or strong): This obviously depends a bit on what the new build would look like, but it seems from what we've seen in the single player that it would be something of an amalgam of classic (SCI-SCIV) lizardman and SCV Aeon, minus the bizzare magical features. This increases the odds over a pure classic approach, which would introduce a second sword and shield character (or a third, if Cassandra should be included).
Popularity (moderate): Lizardman is not a top-tier character demanding the adulation of a huge legion of fans who have mained him. But this has not stopped his being included in every main sequence game in the series for nearly two decades, and he does have his advocates. Honestly, he may be the most obvious absence from the roster at this point, with respect to making the game representative of "classic" Soul Calibur.
Cassandra:
Continuity (strong): Cassandra was introduced in SCII and featured prominently in SCIII/AE and SCIV/BD. She plays notable roles in the single player narratives, particularly Sophitia's Soul Chronicles story.
Uniqueness (moderate): Cassandra has her own vibe, but there's no denying that she fits in the same basic niche as Sophitia, with a close overlap in range, speed and other aspects of her mechanics. Still, she would make only be the second sword and shield character in the game, where historically these styles have been very strongly represented.
Popularity (strong): Cassie is the single highest character in the popularity poll who has not yet been added to SCVI, and this is one area where I am sure it is more or less accurate--if anything, I'd say her popularity as a fan favorite may be even higher than where she landed on that list. Her absence from V (and VI thus far) has been highly noted and commented upon.
Yun-Seong:
Continuity (strong to moderate): Yun-seong entered the franchise relatively early (in SCII) and was a staple until SCV. He is featured or referenced in both SCVI story modes, and he has strong connection to a number of main characters.
Uniqueness (strong): Presuming Hwang is not added to the roster first, Yun-seong would be the only dao wielder.
Popularity (strong to moderate): Yun-seong is a little bit of a fan favorite with a dedicated following in SCII-SCIV. It's pretty difficult to discern whether he would be more likely than Hwang, but I'd wager at least one of them is a certainty.
Hilde:
Continuity (moderate to low): Hilde was introduced in SCIV and has not had a representation in SCVI's story mode thus far. However Wolkfrone and other story elements connected to her did feature in both single-player modes. Somehow she does indeed feel like she would fit easily enough into the roster of VI; at a minimum I judge her much more likely than Algol or Dampierre.
Uniqueness (strong): Hilde has a distinctive style that isn't particularly like anyone else in the franchise.
Popularity (moderate to high): Hilde had some pretty loyal devotees in SCIV (where she was often regarded as top-tier, almost to the point of being broken) and V. She comes in just before the half-way point of the popularity poll.
Setsuka:
Continuity (moderate to low): Setsuka was only introduced in SCIII, but this is also true of Tira and Zasalamel, who both feature in SCVI already. However, there is very little in the way of lore anchoring her into the game (in any mode) so far.
Uniqueness (strong): Setsuka has a play style that was only ever partially replicated by Alpha Patrokolos. She would definetly have a sharp contrast and distincive flow when compared against the rest of the roster.
Popularity (moderate to high): Setsuka was always very techy and took a high degree of dedication to learn, but she is another character with a very dedicated population of players who mained her between the two games she previously featured in.
Li Long:
Continuity (moderate): Li Long is a SB/SE original who managed to just eek into SCIII as a bonus character. However, he is present in the Libra of Soul story mode in SCVI and definetly exemplifies the old school SC vibe.
Uniqueness (moderate to low): Li Long doesn't just look quite a bit like Maxi, he plays very similarly too. that said, they are the only to nunchuku characters in the franchise, so this similarity does not entirely preclude him.
Popularity (moderate to low): Maxi's greater consistency of inclusion since SCI has led to his being the premiere nunchuku style and with so many newer and younger players being wholly unfamiliar with Li Long, the demand for his inclusion is not high.
Rock:
Continuity (moderate to high): Rock is a classic character and was a stalwart of the first five games. However, he is all but completely absent from representation and reference in SCVI.
Uniqueness (moderate): SCVI does not have a whole lot of slow and heavy hitters as yet (others may correct be with a detailed fram analysis, but I believe Nightmare and Sigfried haven't been this fast, relative to to the general speed of the game they are in, since SCIII, if ever, and I bet 2B ends up as fast or faster), so in that sense, Rock would help round-out the roster some. On the other hand, some of his moves/niche have been very obviously migrated into Astaroth in recent games, and even before this the two have been viewed as largely redundant on one-another.
Popularity (moderate to low): Rock is widely regarded as being a low-tier character in the last two entries he featured in and his popularity sank relative to that of Astaroth, who always had a more eye-catching style anyway. Not many people are clamoring to see Rock return, but frankly I think the big guy is very iconic of the early days of the franchise and I'd personally be happy to see him back.
Algol:
Continuity (moderate to low): Algol was introduced in SCIV and although he functioned as the boss and was at the center of the narrative for that game (such as it is) he nevertheless had a bit of a tacked-on feeling to him. Nevertheless, the "Hero King" is mentioned repeatedly in SCVI's single player narratives, with a number of repeated references in Zasalamel's summary of the Soul Calibur backstory--almost in a way that seems to be prepping Algol for introduction, though arguably for a follow up game rather than DLC.
Uniqueness (moderate to low): Algol obviously is redundant (in visual style anyway) on Azwel, a character just introduced in SCVI: they both use weapons which are said to be powered by the essences of Soul Calibur and Soul Edge, and which spring into existence at will, taking various shapes. The similarities are arguably superficial and there are substantial differences in the practicalities of their fighting styles, but even the surface-level similarities could be strong enough to ground Algol's inclusion.
Popularity (moderate to low): Algol never really connected with a substantial portion of the community and is largely viewed as gimmicky, despite having a mid-tier playability. I think we can safely assume fans would less nonplussed by his being ommited than just about anybody else on this list.
Dampierre:
Continuity (low): Dampierre was introduced as a new character in Broken Destiny and featured as pay-for DLC in SCV. He's not particularly crucial to any plotlines (certainly not early ones) and he hasn't been featured or mentioned in SCVIs story modes to date.
Uniqueness (moderate to strong): One thing you can say for Dampierre is that he is not exactly redundant on anybody. There's definetly a bit of Voldo in the DNA of his mechanics, but on the whole Dampierre is definetly...his own thing.
Popularity (low): Dampierre is lower on the popularity poll than any of the other characters listed here (and of all characters in the poll, only mimics, bosses and some SCV new faces rank lower); I would say his place in that poll is accurately indicative of the overall degree to which he was embraced by the community. To some degree, this may be more a reflection of how he was introduced to the series (in the only portable game in the franchise first and then later as paid DLC in SCV when the franchise took a mini nose-dive; none of V's new characters have remained super popular) than it is about anything intrinsically flawed in his style. Sure, he's a bit of an oddball style, but no more so than Voldo certainly. But regardless of the cause, I don't see a petition for his inclusion starting any time soon.
Beyond the returning regular cast, I have a few suspicions on who may potentially feature as a guest characters, should any more be announced. For starters, I really would bet dollars to doughnuts on Kratos; he has the right look and fighting aesthetic for Soul Calibur (which is why he was already adapted once), he's ripe for cross promotion now and his updated story, moves, and look all justify repeating him, with a substantially updated moveset. I'm tempted to say with the Final Fantasy VII remake on the horizon, Cloud could be an option, but I bet his licensing is tied up by Smash Bros. for at least this generation. I think Noctis would have been an awesome choice as well (and I think Azwel is the result of developer's remorse on the part of Team Soul after Namco wasted such a great asset on Tekken, where he doesn't fit in remotely as well) but I doubt they are going to leverage the same guest character for both franchises. That's a pity though, since it would have also made Algol a less awkward addition, had Noctis been included instead of Azwel.
Ryu Hyabusa of Dead or Alive/Ninja Gaiden fame (or maybe an ancient same-clan analog, similar to how Yoshimitsu's dual-franchise situation is explained) has always been a cross-over I'd like to see, but I don't know that I've ever heard of a Namco/Tecmo cross-promotion or joint effort of any sort, so that's probably out. While we're on the subject of things I think would fit in well with this entry but will never happen, I never understood why Gabriel Bellmont and/or Alucard were not tapped for V, but the Lords of Shadow sub-franchise is so past the height of its (kinda limited) popularity that I don't see it happening now. I actually never played those games beyond some demos, but the aesthetic similarity and move potential is immediately striking to me. In a similar "the moment has probably passed" analysis, I think if SCVI had come out a little earlier or Horizon: Zero Dawn a little later, we might be seeing Aloy instead of 2B. If we should see a surprise reveal of H:ZD2 in 2019 though, I wouldn't rule her out as an entry, given the cross-promotion potential.
Just to cover everyone remaining: I don't see any reason why other mimics aside from Inferno cannot be added to the roster: Charade, Edge Master, and Olcadan being the obvious choices. It would be cool if these skins shared a button with Inferno and were just added to the pulldown menu that usually is utilized for colors on the select screen, as an update gimme; it would take trivial resources for Team Soul to create these skins and add them as default. I'm sure it goes without saying that there would be a furor if any mimic were added as part of a season pass unless that pass already had the number of characters established in the first season (four) or more. I wonder if Night Terror or Abyss may feature in a story mode at some point as well, but that seems like more development effort than is likely to be devoted for a feature that will not be a strong selling point to the multiplayer crowd who are going to be the only ones buying and advocating for this game in a year's time; I suspect they may be more likely as a SCVII feature.
That's the situation as I see it in a nutshell. I'm pretty confident that the remainder of the current season pass will come the topic six characters in the list above, presuming no new characters or new guest characters (also probably a safe bet).
