Who needs the failed Fantastic Four when you have access to the entire X-Men library of Marvel Comics stories and characters? That's the position Twentieth Century Fox is in and the answer is Marvel Studios does! That reality, where Marvel very much desires access to many of the alien races and other-worldly characters that Fox currently holds the license to came to a head in 2012 when Fox's hold over the Daredevil property was coming to an end. Marvel reportedly attempted to work out a deal that would allow Fox to keep Daredevil in exchange for the rights for Marvel to use Fantastic Four characters Silver Surfer and Galactus. It was a no-go.
Since then, Marvel has made efforts to push the Fantastic Four out of the spotlight in every possible way. Fantastic Four is not allowed to be licensed out, their two comic series no longer exist, posters and art were taken down inside of Marvel HQ, and they've even been removed from the latest official Marvel Encyclopedias. Any relationship between Fox and Marvel that previously saw Fox's Marvel-branded movies promoted on Marvel.com, and Marvel employees on the sets of X-Men movies was seemingly in the pits. That all changed recently though.
Even though Fox holds the live-action rights to the Fantastic Four and X-Men libraries, they still included Marvel Entertainment in their dealings for the rights to make a pair of live-action X-Men TV series. Given the abysmal failure that was Josh Trank and Simon Kinberg's Fantastic Four reboot this summer, speculation pointed to the obvious: Marvel and Fox could feasibly engage in a partnership that would allow Fox the ability to make X-Men TV shows and Marvel could finally make use of the Fantastic Four property. There was one big report that made headlines saying exactly that had happened after Marvel and Fox announced a partnership for Legion and Hellfire (around the same time Marvel added new release dates in 2020 no less) but it was denied on all fronts.
Meanwhile, producer Simon Kinberg - who's the boss and planner of Fox's own Marvel Cinematic Universe - continued to say that Fox is still interested in the Fantastic Four characters and that he's still planning away at some sort of Fantastic Four sequel. We posed the question of why since in the grand scheme of things, the Fantastic Four restricted to a universe of characters that Fox has the rights to can never be as interesting or successful as the Fantastic Four as they would exist in the actual Marvel-owned Marvel Cinematic Universe, where the likes of Reed Richards can interact with Tony Stark, Black Panther, etc. And no one wants to see another rushed reboot, so that's not even an option for Fox.
Fox now sits in a very similar position to that of Sony Pictures with the Spider-Man franchise. After The Amazing Spider-Man 2 failed to meet its exceedingly high expectations critically and financially in Sony's rushed effort to create a little movie universe with spinoffs of their own, and after leaked Sony emails revealed that they had been in talks with Marvel over sharing Spider-Man, they fell into a trap. Continue to make Spider-Man in isolation, or work with Marvel to let him exist in a world with other superheroes as he does in Marvel Comics. They chose the latter and now Spider-Man is going to become a face of the MCU beginning next year in Captain America: Civil War.
The Fantastic Four absolutely need something similar since the brand has been systematically crushed. Fox will never have Marvel's backing on future Fantastic Four movies or spinoffs so the only way FF movies reach "event" status is by working with Marvel, to let those characters exist in the larger, established universe, and to have Marvel start to push merchandise, comics, and tie-ins once again. All of this is especially interesting now since yesterday it was revealed that Fantastic Four 2 no longer has a release date.