This report from Latino Review suggests that 20th Century Fox is actually seriously kicking the tires on the idea of crossing over two of their comic book movie franchises down the road (ie maybe around 2018):
If you already happened to take a look at my column about DC’s possible San Diego Comic Con plans to introduce two Green Lanterns, you might have noticed I put quotes around “unknown” when discussing 2018’s X-Men movie release from Fox. The better descriptor would be “undecided,” because 20th Century Fox has looked around and noticed what Marvel and DC are doing.
If Josh Trank’s reboot of Fantastic Four can manage not to suck and Fantastic Four 2 in 2018 isn’t a complete disaster in the production phase AND assuming X-Men: Apocalypse doesn’t nose dive the entire franchise, the current plan is to have 2018’s Fox comic book movie be a crossover film that features the X-Men and the Fantastic Four. Presumably they would keep Trank’s FF cast, though who would comprise the X-Men is still as unclear as when Channing Tatum will give us some real details about Gambit.
This is far from a done deal and Fox isn’t going to count their Fantastic X- Chickens before they hatch, but they have to think that crossovers is where the money is at three years from now. (source: Latino-Review.com)