I think the next team up film is probably going to be Avengers vs X-Men in Phase 5. They're making a big play out of the multiverse, and I'd guess that at some point we're going to see some of the alternate earths combine. That'll bring X-Men and mutants into this universe - obviously there'll be a conflict with the Avengers at first, before they unite to take on a bigger threat. I think they'll shy away from doing solo X-Men movies for a few years more, instead putting those characters into other properties.
Seen some suggestions that Fantastic Four could be a 60s set period piece, which would be fun. Although presumably if they did that, they'd be brought into the modern era in some way?
