Fox is currently filming The Fantastic Four in Louisiana. While it's in production, writer/producer Simon Kinberg has been teasing it here and there.
Kinberg has already went public saying that The Fantastic Four is in their own movie universe and the crossover potential with the X-Men is not happening. He has also mentioned that in order for Marvel's first family to truly be fantastic, then mutants cannot co-exist in the same movie universe because they are a part of evolution—which can be seen as being more fantastic than the members of the Fantastic Four. This makes sense.
There could be issues with teasing the X-Men in The Fantastic Four movie universe in case the movie is a flop. Obviously that's speculation but the previous two Fantastic Four films were not received well and didn't knock it out of the park in the box office either.
Whatever the reason(s) for The Fanastic Four being in its own movie universe, it seems like Fox is being cautious by not blending it into the X-universe, just in case it fails in the box office. Will it fail? Not sure, but having a strategy in place to see how the reboot does financially is smart.
Plus, in the comic universe, the Fantastic Four like to travel between different dimensions so it can easily be written into a sequel that the Fantastic Four come across a parallel dimension where the X-Men movie universe exists and the mutants need the help of the Fantastic Four in an potential crossover movie.
And there you have it. That's how Kinberg can say The Fantastic Four is not in the same movie universe as the X-Men right now but can still have a crossover film with both in the future. You're welcome.
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