Originally Posted by Zarex
Of course I am making an assumption - the same assumption people are making when they assume that these two separate character families can be crossed over without Disney's approval. Disney/Marvel can raise the argument, in court if necessary, that they did not sell the character families with the intention of their being merged, and that doing so has the potential to harm their future viability as separate entities.
My own assumption would be that a crossover should not pose a problem.
Here's my reasoning: There have been comic book miniseries featuring X-Men and Fantastic Four together. Dr Doom was the villain in an X-Men comic story. Storm has been a member of Fantastic Four. Wolverine featured in one temporary line-up of Fantastic Four. Mr Fantastic and Susan Storm have a child who is a mutant (Franklin) and who featured in X-Men stories including Days of Future Past. Thus, there is already precedent for crossover in the source material.
Any cinematic crossover would be way down the line, in any case. First they need to successfully reboot the Fantastic Four for at least a couple of films before considering a crossover adventure with some/all of X-Men.