The trailer for Marvel vs Capcom 3 has been released today, and while Deadpool does not appear in the game footage many fans are speculating that Deadpool is in the game. The cause for their speculation is a distinctive silhouette with the Marvel logo.
There aren't many details about the game yet, so a full list of characters hasn't been revealed. [SOURCE]
UPDATE: Kotaku has also posted their speculation on the cast of characters, including Deadpool. [SOURCE]
