I'm a very one-player oriented fan of the series; like several people here, one draw besides the obvious fighting game aspect is the story-driven element behind the Soul series, and each character's interaction with the main swords. This is the Soul Calibur I've known to love; each character is fighting to either obtain or destroy Soul Edge, even if it means having to sacrifice something. This is what made the games prior to IV so charming -- it wasn't afraid of its absurdity, but rather capitalized with with either full CG cutscene endings (with input endings in SB/III for extra flavor), or the lovely sepia-tone illustrations of the second game. The story mode itself could use a bit of upgrading itself; SCIV introduced an interesting development in the form of a "tag out" mechanic which added an all new element of strategy, and would be a welcome addition as a new game mode as well as an implement in the upcoming game's story mode.
In SCIII and IV, I felt the games were too emphatic on Zasalamel and Algol, respectively; in my opinion a game with as many interesting characters as Soul Calibur should not be too heavily focused on one character (although they make excellent mid-bosses), and return to the SCII method of a normally unplayable character being the game's end boss. Inferno is the embodiment of Soul Edge, the game's true antagonist, and I think it should stay that way for all characters (even if it creates some paradoxical scenarios like Nightmare, although in his case Inferno could be a separate part of Soul Edge mutating away from the actual sword, and Nightmare wants to absorb him to obtain full power and even bring down his reign as Night Terror once more).
I'm obviously not deeply in-tuned with the game's inner workings and mechanics; I'm more interested in the aesthetic and storytelling purposes of the games, the distinct cast of characters and intentions which makes Soul Calibur stand out in a world where fighting games are filled with clones and clichés about some supreme tournament everyone's drawn to (sorry, Tekken). I kinda want that element back in the game, in which the final boss is some form of Soul Edge (Inferno comes to mind, give or take as you will), with vague character-centric endings based on what you choose for his/her path upon defeating this boss and dealing with Soul Edge/Calibur.
Characters! I'm not picky about such, though I've grown fond about the mainstay cast; certainly Amy, Hilde and Dampierre fit somewhere in there. Though I haven't played much of IV, from what I understand they could use some retooling to turn them down some and make them fit in easier with the other characters, but Amy, Algol, and Hilde add their own special allure to the franchise and Dampierre seems absolutely hilarious and entertaining; not quite a joke character, but a tricky and unpredictable one who gives off the illusion of a joke but can actually be quite dangerous.
Returning/new character suggestions are pretty similar to everyone else's. Hwang, Li Long, at least one of the ditto fighters (or even introducing a new one altogether, but I feel as though Edge Master/Olcadan have been left out, and the fact that people legitimately enjoy the challenge of Mokujin-esque fighters as their main character), maybe Arthur, maybe Bangoo, since seven years have passed story-wise since Soul Blade and he looked 8-10 way back when, perhaps he's ready to return from his quest for manhood/coming of age and try his hand at finding Soul Edge to please his father. I don't want to appear more pretentious than I already have by suggesting too many brand new weapon styles, but I'm certainly not adverse to the concept of new characters, either.
Guest characters, I also have no originality for. It was kinda nice not having to bother with such in III, but then again they're also a major money draw in spite of the fact that I would rather have more emphasis on the mainstay, canonical cast. in the end it's not really my decision, though story-wise the central characters are where much of the canon should reside. In SCII, the guest characters and Necrid each returned to the place they had come from or sealed themselves away, and this pattern should remain as opposed to the "I'M HERE, NOW LOOK AT ME" draw to Vader/Yoda in IV. Console exclusive guest characters are somewhat iffy, but bringing back some old faces could be possible.
I understand the Wii isn't as powerful as the 360 or PS3, but at the same time the GC version of Soul Calibur II did exceptionally well thanks to Link, turning new heads to the series and, for the time, was rather beautiful graphically. Maybe I'm just biased, but a more bare-bones release for PS2 and/or Wii with Classic and GameCube controller compatability (with all of the characters, game modes, stories, etc., but without certain features like creation and customization or without as much emphasis on the high-definition graphical capabilities) would appeal to a broader audience and bring some higher sales from those SCII fans who have been left in the dust since the last console generation phased out or have yet to pick up on the high definition trend. Alternatively, to bring closure as to how he ended up in the SC universe in Legends (unless he was just fanservice), reintroducing Lloyd from Tales of Symphonia would also be acceptable representation hailing from another Namco series.
All characters having their trademark stage which you could view the information for via character profiles were also a nice touch, as were the voice reels/quotes, event viewer, etc. Just make the game as pleasant as a one-player experience as it is two-player, with plenty of moveset and gameplay exploration, inputs, etc., that appeal to both casual and competitive players, and I'll be set gameplay-wise.