Marvel set the blueprint for unified film universes with Iron Man. Since then, they’ve made billions at the box office with every subsequent Marvel-related film tying into an overarching narrative. Some of the groundwork for The Avengers was placed in Iron Man, for example. Keeping to a cohesive narrative structure is something that involves tons of planning and forethought and can’t simply be rushed through. However, some of the other studios aren’t necessarily treating their properties with the same care, rushing through sequel and spin-off announcements like nothing.
It makes sense, then, that a Fantastic Four and X-Men crossover would happen sooner than later. They’re both owned by Fox, and each film property is being written by Simon Kinberg. However, that’s apparently not the case, if Kinberg’s word is to be believed. During an interview with ScreenCrush, the writer was asked point-blank whether or not an FF/X-Men crossover would take place. His answer is somewhat surprising:
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?” […] No, it is, they live in discrete universes.
The logic behind his reasoning is hot and cold. On the one hand, yes, creating a universe that’s already established itself as having mutants would neuter the Fantastic Four’s rise as the premier superhero team/family. At the same time, this didn’t stop Marvel from creating their universe full of mutants and superheroes. It makes better sense to have each property thrive on their own instead of lumping them all together (re: Batman vs. Superman) and contemplate a crossover down the line. Whatever the case may be, it’ll be at least a few years before we see a Thing and Wolverine Fastball Special.