According to Yahoo! Movies, X-Men: Apocalypse director Bryan Singer has been making moves towards uniting Fox's X-Men and Fantastic Four franchises. During an interview meant to promote the home video release of X-Men: Days Of Future Past - Rogue Cut, Singer said:
“Those ideas are in play,” he says. “That would be a natural match-up because they’re both ensemble films and there is a natural mechanism by which to do it.”
According to the same post, Singer has approached the "brain trust" (those are definitely just normal quotation marks, not sarcastic ones) behind Fox's current Fantastic Four reboot, and that the crossover would "deal with time" somehow. I'm thinking time travel, but of course it's possible that Singer's interested in joining forces with Josh Trank's team for a story about horology, or watch repair.
Admittedly, this story's pretty slight. It ends here.
You are invited to debate whether or not a Fantastic Four/X-Men crossover is a good idea in the comments below (I vote yes, because why the hell not).