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Really, that's the only way I see them being successful under Fox. These days, most people expect all of these heroes, and stories to be connected. The new Fantastic 4 failed because it was it's own standalone thing, always meant to be it's own standalone thing, which I don't think is fair, but I do see that as a legitimate reason for it's poor box office results. And, well, apparently it sucked hard, I'm sure that had a lot to do with it. I think X-Men stays afloat because it lives in it's own little world without having to pull from a billion other franchises, but with so many characters, it can seem just as big, and you always have a plethora of new characters to anticipate. If they add the Fantastic Four characters, they'd be able to replicate the worlds colliding approach of the MCU, which would generate a lot more buzz, especially for the prospects of a new standalone Fantastic Four movie. It couldn't hurt, I can't imagine people just refusing to see a X-Men movie because it has those characters in it.
Also, Doctor Doom fighting the X-Men on the big screen. I don't even have to explain why that'd be badass.