The teaser trailers for Marvel vs. Capcom: Infinite did not show any X-men characters.
Looks like the rumors were true: Marvel vs. Capcom 4 (re-branded as Marvel vs. Capcom: Infinte) will be released in 2017. Sony and Capcom revealed the new title at the PlayStation experience over the weekend, although official details were scant. We do know that it will launch in 2017 for the PS4, PC and (probably) Xbox One.
We also know that it will have Mega Man, Ryu, Iron Man and Captain Marvel, but so far those are the only confirmed characters. X-men were completely absent from the reveal trailer, which is somewhat strange as Magneto is pretty much the de facto mascot of the game. It's possible that due to licencing issues we may not see the X-men in this iteration of the game.
Another big change to the MvC formula is the shift from 3v3 to 2v2 battles. The game will also feature the Infinity Stones from the extended Marvel universe (hence the title) functioning as a sort of out-of-combat way to influence your character's abilities.
It will also feature a "cinematic story" that "answers the questions regarding the new clash and lets players step into the shoes of classic characters from both sides as they wage war against powerful forces in an attempt to defeat a new villain."