Assuming the rules for 3rd party characters from Brawl still apply, Mickey didn't originate in a video game. Ezio is eligible in that he starred in Assassin's Creed II: Discovery on DS. Who really remembers that game though? Connor would make more sense since ACIII was at least a Wii U launch title.
Bayonetta would be cool (and I still think she's a possibility), but one-up than Snake? Her game is niche a best despite the cult following. The only character that would definitely one-up Snake's surprise announcement is Sonic and he's already in. Hopefully, he makes the cut. Mega Man is a possibility, but Capcom keeps trying to convince people that Mega Man has no audience. A Resident Evil character might do the trick. Jill looks pretty awesome in Marvel vs. Capcom 3 so there's definitely a moveset for her even if they remove her gun moves. As much as I love Jill, I'd still go with Mega Man personally.