We got Nash back. Don't need another character like that.
If the rumors are true about the game once again having 16 characters (just like how SF4 started out-- the arcade version had the original 12, plus the four new ones), I hope they go for a more varied roster of returning characters. But I expect to see more characters get released as DLC.
Also really, really hope the returning roster isn't SF2 (with Nash replacing Guile) again. Take from other games. Bring back characters who weren't in SF4. We got Ryu. We got Chun. We got Bison. Throw in a grappler like 'Gief (would prefer Hugo again) and maybe put Cammy in there and that's enough from the SF2 era. If they want to bring back new characters added to SF4 though, that's cool with me. Abel, Juri, Poison, etc.? Sure. Gimme R. Mika back too-- in fact, give her a 360 and put a sole female in that slot this time around.
I bet Alex is going to return, too. He was in Tatsunoko Vs. Capcom, but that died as soon as MvC3 came out.