Ever since their X-Men franchise was revamped and the Fantastic Four reboot was green lighted, rumors spread that the two franchises would meet up in a grand crossover. However, this was something that Fox, through Simon Kinberg, tried to downplay the idea to the point where it seemed highly unlikely to happen.
And then we get this from Bryan Singer as he promoted the X-Men: Days of Future Past-The Rogue Cut
So an X-Men/Fantastic Four team up is in the works, at least at a small level. I’m wondering if the rumor of a shared universe with Marvel Studios also has a bit of a fire to go with its smoke. A lot will depend on how well Fantastic Four does next week.