Reportedly, a NeoGaf user by the name of Ryce (who is considered on the board to be a reliable source of inside information) revealed what he claimed to be the list of hte inital roster for Marvel vs Capcom: Infinite, which would be the fouth (depending on how you count the Ultimate edition of the third game) installment in one of Marvel's footholds in the gaming world. The results may certainly be surprising, so there they are:
CAPCOM
Arthur
Chris Redfield
Chun-Li
Dante
Firebrand
Jedah
Monster Hunter
Morrigan
Nemesis
Ryu
Spencer
Strider Hiryu
Megaman X
MARVEL
Ant-Man
Captain America
Captain Marvel
Doctor Strange
Gamora
Hawkeye
Hulk
Iron Man
Nova
Rocket/Groot
Spider-Man
Thanos
Thor
Ultron
One thing you may notice about this roster is that there aren't a lot of newcomers this time around, mostly consisting of older returning characters (though the "Rocket/Groot" entry indicates that, while Rocket Raccoon was in the previous game, he has been redesigned into a two-person fighter partnered with Groot).
More conspicuous however, is not who is new, but who is missing: The X-Men. While various rumors continue to swirl about Marvel's treatment of properties who's films belong to 20th Century Fox, it seems this will do nothing to quell the speculation, as the X-Men franchise has historically been a longstanding staple of Marvel vs Capcom. In fact, the entire Marvel vs Capcom series is the spiritual successor to 1996's X-Men vs Street Fighter.

While it may be possible that the X-Men may return in the form of DLC, or that perhaps this list is entirely innaccurate, it seems we'll just have to wait and see for an official announcement, possibly at the upcoming E3 presentations. Till then, leave your thoughts on what you think of this speculative roster in the comments below!