Fox was planning an X-Men/Fantastic Four crossover movie several years ago - but here is why it never happened. The studio enjoyed various levels of success in the early 2000s with the launch of their X-Men and Fantastic Four franchises. Despite owning the two big Marvel teams, they never brought them together on the big screen. They completed an X-Men trilogy in 2006 and saw Fantastic Four end after two films in 2007. With both of these separate franchises fizzled out, the studio was forced to think of a new way to continue them.
In the summer of 2010, Fox announced how this would happen, as Matthew Vaughn signed on to direct X-Men: First Class. The movie would be set in the 1960s, and based on a script by Zack Stentz and Ashley Edward Miller. The film served as a soft reboot of the X-Men franchise that brought in a talented, younger cast to play familiar roles. It was here that audiences met James McAvoy's Professor X, Michael Fassbender's Magneto, Jennifer Lawrence's Mystique, and so on. Recently it was revealed that, at this same time, Stentz and Miller were working on a top-secret script that would pit the X-Men and Fantastic Four against each other.
The X-Men/Fantastic Four movie was in the works in 2010 according to recent reports, which could have made it the direct followup to X-Men: First Class. Initial plot details for the film saw it open with Johnny Storm trying to apprehend Molecule Man, but then going nova and causing great destruction in Manhattan, which kicks off a movement for the Superhero Registration Act. This would have split the X-Men and Fantastic Four up on different sides, basically making this Fox's Captain America: Civil War - but several years before. According to Stentz, the film would've even featured Deadpool and Daredevil. The two differing viewpoints would lead to Wolverine and Mr. Fantastic (among others) being on opposite sides and fight one another.
To bring this story to life, Fox reportedly courted Paul Greengrass to direct the crossover. At this time, Greengrass was still high off of the Bourne franchise and would be coming off of his latest Matt Damon pairing in Green Zone. The director didn't begin working on his next project, Captain Phillips, until the summer of 2011, so this could have been the same possible window of opportunity for a Greengrass-directed X-Men/Fantastic Four movie. However, the film never moved beyond the early development phase, and the success of X-Men: First Class appears to be the biggest reason why.
Fox was happy with the results of the movie, even if it is actually one of the lowest grossing entries in the franchise, and used it as the template for everything that would come after. They tossed the Fantastic Four crossover idea aside and instead put all of their attention on doing X-Men: Days of Future Past as their big crossover movie. The film brought together the new cast and the original actors for a franchise and continuity-altering adventure. The execution was rushed according to what Vaughn originally had planned for the story, which is why he did not direct the movie. Still, the time-traveling film was a big hit - its the highest grossing entry of the main saga worldwide - and resulted in X-Men: Apocalypse hitting theaters two years later and now Dark Phoenix.
So, it appears that First Class's success and the overall timing of the project is why the X-Men/Fantastic Four movie never happened. There are plenty of details that are not clear about the film's placement in the larger universe, such as if it would also be set in the past with the First Class cast. Stentz says he wrote a script for a Fantastic Four reboot movie too, so it possible that they could have had a solo movie first. In either case, the X-Men/Fantastic Four crossover movie was just the beginning. The movie would have reportedly had a post-credits scene teasing a possible Secret Invasion storyline that could have been a multi-film story for the universe. Instead, fans will have to wait for these meetups and crossovers to happen as part of the Marvel Cinematic Universe now.