The "X-Men: Days Of Future Past" timelines opens up a lot of possibilities. One of them could lead to a grand crossover with the Fantastic Four despite the fact that they don't exactly share the same universe.

X-Men and Fantastic Four screenwriter Simon Kinberg is slated to be the main man in lieu of Bryan Singer at this year's Comic Con if the legal issues of the latter continue to be a hindrance. Kinberg is the most integral part of the brain thrust on both movies so he is qualified to clarify the issue of a crossover.

It has been surmised that Fox would eventually bring their superhero groups together and the time travel angle that they used will serve as the floodgate for that possibility. He cautions on Screen Crush:

"Well, it's complicated. Because none of the 'X-Men' movies have acknowledged the notion of a sort of superhero team - the Fantastic Four. And the Fantastic Four acquire powers, so for them to live in a world where mutants are prevalent is kind of complicated, because you're like, "Oh, you're just a mutant." Like, "What's so fantastic about you? [...] So, I guess we'll go to school now. We'll go to Xavier's school." No, it is ... they live in discrete universes."

Screen Rant elaborates the idea:

"When he says "they live in discrete universes," it seems pretty clear - there won't be any crossing over for these superhero teams. But then there's also the talk of how "complicated" it is. Is it possible that Kinberg just means that the Fantastic Four will remain in a "discrete" universe for the first film, where they can set everything up before eventually crossing over with the X-Men down the line? Or maybe discussions are ongoing as to whether or not a shared universe will be set up? Your guess is as good as ours."

After the success of "X-Men: Days of Future Past" Fox has been rumored to make more sequels and a list of possible spin-offs. An intriguing item is that they have an 'unnamed Marvel movie' set for release on July 13, 2018. It's already a year removed from 'Fantastic Four 2' so there is a possibility that it's for a crossover.