There's also word going around that the roster size will exceed expectations.
Let's hope. Somewhere between 32 and 36 characters in the base game seems like a reasonable hope.
I've posted my hopes list before, but as far as what would make me really, really happy regardless of anything else, character-wise:
Capcom: A Power Stone character, one of Hayato or Joker, a non-Batsu Project Justice character, Firebrand, a male Darkstalker, at least one Phoenix Wright character (Edgeworth or Fransziska Von Karma is fine)
Marvel: A Runaway (preferably Arsenic and Old Lace as one whole character), A non-symbiote Spiderman villain (preferably Doc Ock), Luke Cage
I sure hope that we get a Power Stone stage at the very least - the stages from Power Stone 2 are dynamic, and one of those might be a good choice. A Cannon Spike-themed stage would be sweet as well.