Back when Fox had control of a number of Marvel properties, there was talk of an X-Men vs. Fantastic Four film, as confirmed by X-Men: First Class screenwriter Zach Stentz. The film would have used every Marvel character Fox had at the time and be based on Civil War comic, pitting the two groups against one another after the Human Torch causes excessive damage in Manhattan, leading to the Superhero Registration Act. You may remember a similar adaptation in 2016’s Captain America: Civil War.
Speaking with the Happy. Sad. Confused. podcast, Jason Bourne director Paul Greengrass confirmed that he was in discussions regarding the film, though nothing ever came of it. Oh what could have been. (h/t ComicBook.com)