I don't think they'll have to. I think Fox will either let their rights expire which will take a few years or if they still want to make money off the franchise, they'll offer to do the a similar deal that Sony had done with Spider-Man.
Aside from the new FF movie being terrible in every way, Marvel themselves also have a lot to do with the failure of the movie. After hearing what direction Fox was going with, they got fed up. They cancelled the Fantastic Four comic series and refused to do a toyline to promote the movie or the franchise in any way. They've also done similar things with X-Men but not to this extreme. Just go look for Marvel Comics merchandise in any store. You won't see any Fantastic Four merchandise and you'll fine very little of X-Men.
Marvel wants these franchises back and they're willing to kill them to get them back. Although I don't think they'll want Fantastic Four back just yet. Fox has made that franchise near worthless. A few years, maybe more have to pass.
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