So why not rescue those characters by placing them in X-Men movies?

There may, of course, be practical reasons why this cannot be the case — perhaps restrictions on the usage of particular characters under specific licenses, or a clause simply demanding that the FF and X-Men characters are never to be mixed — but, if not, it seems like a solution to a number of problems for Fox, beyond simply answering the question "Is there a way we rescue Annihilus and the Negative Zone from cinematic purgatory?"

Mixing the two properties would, obviously, expand the scope of the current X-Men franchise considerably, and in the direction the franchise needs it: bringing in new threats to the good guys, and bringing in different ways in which the good guys can be threatened.

To date, after all, the majority of the X-Men movies have the same exact uber-plot — "there is a threat to mutantkind, Charles Xavier and whoever his students are have to deal with it somehow, and Magneto will find some way to be involved in a morally ambiguous manner" — and, to be honest, it's getting a little old, 17 years into the franchise. Bringing in characters like Doctor Doom (a good, old-fashioned despot who wants to rule the world) or the Psycho Man (an alien scientist who likes to literally toy with other people's emotions) would only broaden the stories being told in the franchise and, arguably, broaden the appeal of the franchise at the same time.

It could also differentiate the different strands of the X-Men property, giving each an identity allowing them to succeed in the same way that Deadpool had a fresh appeal purely by avoiding the mutant plight altogether: New Mutants could be a movie series dealing with goofy threats like the Impossible Man (Hey, there's comic book precedent for such things), while X-Men deal with the traditional mutant survival angst and threats. (Gambit, should that movie ever be made, can teach the Mole Man about romantic relationships; it just seems fitting.)

Using the Fantastic Four-related villains and supporting characters in this way would let Fox clear the way for a third attempt at a Fantastic Four movie, should the studio want one, by building an audience appetite for it, while also letting filmmakers work out just how to approach it in the "safer" environment of a successful franchise. If done right, it could be something that would ultimately enrich Fox's larger superhero plans, and jump-start a franchise that has never fared well at the box office. So…who do we have to talk to to ensure that there's a Fantastic Four presence in the next big X-movie?