I think that this will depend on how much say Marvel gets in the roster. When MvC 1&2 came out, the X-men were Marvel's biggest franchise. Today, the Avengers line sells much more and will be featured in a big movie in 2011. If Marvel get their way you'll almost certainly see many Avengers, particualrly Luke Cage and Spider-Woman, and probably Hawkeye (I know Clint Barton is currently Ronin, but I bet that by the time this game comes out he'll be back rocking the purple mask), all of whom Marvel is pushing really hard.
Even within the X-men, the popularity of characters has changed. I bet they'll have Emma Frost or X-23, as well as Deadpool.
Also, if Marvel has their way, I guarantee we'll see Red Hulk in the roster.