This will probably take me ages but I can't resist things like this.
I'm going to go from what we already know about the game, plus who I think probably will/should be in the game (based on the original SC cast and characters from future games that would make sense) and some personal choices mixed in.
1: Sophitia
2: Mitsurugi/Arthur (could occupy the same slot and be switchable with a button. Either they could each have their own style or Arthur could be a remodel and revoice of Mitsu)
3: Edgelord
4: Edgelord Papi
5: Kilik
6: Maxi
7: Xianghua
8: Cervantes
9: Ivy
10: Hwang/Yun Seong (for accuracy to the original and the fact I never got to play it, I'd rather have Hwang, but I also think they could just share a slot similar to my Mitsu/Arthur idea)
11: Rock
12: Taki
13: Siegfried/Nightmare (again to reflect how the old games were, they could share a slot but this time have two separate fighting styles).
14: Astaroth
15: Taki
16: Seong Mina
17: Voldo
18: Raphael
19: Yoshimitsu
20: Lizardman/Aeon (I think it'd be cool to have human Aeon as an alternate costume or perhaps a second character sharing the slot. I do NOT think they should replace reptile Lizardman with that though because that would be boring)
21: Zasalamel/Abyss (everyone seems to want it, his style is one of the most unique, and he's old enough to fit, probably, Abyss could be an alternate costume or a separate character sharing the slot)
22: Tira (as Soul Edge's servant, she seems too important to the plot now to keep out).
23: Edge Master/Olcadan/Charade (there should only be one mimic but I wouldn't mind if they had more than one sharing a slot because Olcadan was so funny and Charade looked awesome. Any one of these would be fine though)
24: Guest (my personal choice would be Senua from Hellblade: Senua's Sacrifice)
For DLC:
25: Amy (much as I wish she was in the base game, that won't happen... this is best I can hope for)
26: Talim (honestly I would have put her in the above list if I had room)
27: Hilde (I personally don't like her but lots of people do and her style is popular and unique so it makes sense)
28: Cassandra (not quite important enough to replace any of the base characters, but would probably be a welcome addition).
29: Setsuka (again, I don't personally like her but many others do so it makes sense and adds another unique style to the game)
30: Viola (timeline wise I don't like this choice, but for the sake of another unique style to add to the game, it makes sense, alternatively, she and Amy could share a slot but should still retain their own style)
31: Z.W.E.I (same as above pretty much)
32: Algol (I was running out of characters at this point but given Algol's age, it'd be interesting to see how he looked/fought BEFORE he possessed the swords.
Hope nobody minds the shared slots idea. Ideally I'd like everyone in the game to have their own slots but with some character it'd be harder to justify I guess.
Alternatively, they could bring back the 'Bonus Characters' thing that SCIII and SCIV had and place some of the similar characters like Arthur in there.
I would also love to see Inferno and Elysium return. Now that Soulcalibur actually has a living form like that, it would be epic to have both be in a game together, probably as unlockable bosses. They would either have to have their own style or take up the Male/Female mimic slots (probably removing Edge Master unless they share with him). Also, Elysium would need to be redesigned to not resemble Sophtia (unless they want some comedy interactions...) and have a less human form (but still resemble the heavenly image to opposite Inferno's hellfire image).