Giving another hint at which fighters will be in the game, Frank Tieri, who's the writer for Marvel vs. Capcom 3, had an article posted on Capcom-Unity explaining a bit about his recent work.
There isn't much that's going to rock your world here, but there's one interesting quote. Tieri was asked which characters he enjoyed writing the most, and he had this to say.
"As for Capcom, I’ve always been partial to the Resident Evil and Street Fighter franchises so it’s probably those characters, especially the villains. They’re just such evil sons of *****es I had a ball writing for them."
There aren't a ton of characters that are considered villains in the Street Fighter series that have enough popularity to warrant being put in Marvel vs. Capcom 3.
As such, some people have suspected he's talking about M. Bison here, although there are a few other notable evil fighters that have shown up in the series.