I would love to have Black Widow, Kamala Khan, Medusa, Void, & Nico Minoru join in on the fun. If the Marvel VS. Capcom series needs one thing, is a better mixture of heroes and villains. Ultron seems like a shoe-in, but I would like a more left field villain that would fit in. She-Hulk was fun to play as in MVC3, so I hope she returns.
In the case for the Capcom side of things, Classic Mega Man needs to be brought back as the rep for the Classic Mega Man series, with a revamp moveset that takes advantage of his abilities from the series he hails from.(They can do it in Super Smash Bros., so there's no excuse this time around.) Captain Commando, Edward Falcon/Rouge/Wang-Tang, Samanosuke, Batsu Ichimonji/Kyosuke Kagami, Tessa/Leo, Gene, they can't go wrong with having a nice mixture of the new and old for their side of the roster.