I predict Billy Hatcher will make a return, maybe even Ryo. I would like Blaze the Cat and Vectorman, but both of them have pretty much been deconfirmed. Other than that, I see no reason to not include at least one Streets of Rage character.
Zombies are like sprinkles, they make everything sweeter!
Blaze won't be showing up as a racer. The Sonic roster is locked and there'll be no more.
As for my speculation, I honestly don't have anymore I want. I already got B.D. Joe as it is, and Wreck it Ralph has my attention enough that I'd like to main him as my secondary. I guess my only other choices would be Segata Sanshiro and Sketch Turner.
http://backloggery.com/SetnaroX "Proper sound shatters glass. Proper sound shatters the Universe."
Pretty sure they said 23 characters, and 23 have been revealed http://www.tssznews.com/2012/10/23/several-previews-reveal-more-info-on-sonic-all-stars-racing-transformed/
Either way, I'm just glad they have Vyse, NiGHTS/Reala (even though they messed these two up), pissed that they added Gillius over Tyrus, ticked that there's still no Toejam and Earl, despite all the effort Sumo, and the TJ&E creators put into trying to get them into the first game, no Phantasy Star characters, and no Streets of Rage characters. Hell, there are tons of characters getting left out. Bayonetta, Resonance of Fate, Valkyrie Chronicles, Sakura Wars, Shining Force, Panzer...okay why is Sega a dying franchise with all of these under their belts? Seriously, Sega Smash Bros time... But anyway, primary, I'm happy that they have Vyse.