Distinct universes

The fact that the Fantastic Four and the X-Men live in distinct universes is not such a big deal "“ after all, some sci-fi mumbo jumbo has already made travelling between the universes possible (as seen in the trailers for Fantastic Four). So who is to say that, in a crossover, Doom won't travel to the X-Men universe to take it over, with Fantastic Four jumping in to help the X-Men?

For the moment, Fox (while owning quite a few superheroes and supervillains) hasn't released yet its full slate of comic book movies. All we know is that they will make Apocalypse, Deadpool, the Wolverine sequel, Gambit, and the New Mutants. Plus, of course, the already announced sequel to Fantastic Four (due in 2017).

If everything goes well, 2018 is the best year for the crossover to happen.