During Marvel's financial troubles in the '90s, the publisher sold off the film rights to many of the iconic heroes that Stan Lee helped create. And while Marvel Studios now shares the rights to Spider-Man with Sony, no such deal exists with 20th Century Fox, which holds the rights to the X-Men and the Fantastic Four.
Officially, Marvel Studios President Kevin Feige says that it's possible Marvel will regain the rights to the X-Men and Fantastic Four, but that it's highly unlikely. However, Lee revealed at Wizard World Nashville that he believes Marvel is working to regain the rights to those characters.
"Sooner or later, they're going to get the rights back to all our characters," Lee said. "They're working on it, and they're still making X-Men movies and stuff. Don't worry about it. You'll get more Marvel superheroes than you'll have time to look at in the next few years."
The movie rights to the X-Men remain with Fox, but the studio did work with Marvel to change Negasonic Teenage Warhead's powers in Deadpool in exchange for allowing Ego the Living Planet to appear in Guardians of the Galaxy Vol. 2.
Of course, Lee hasn't been involved in Marvel's day-to-day operations for decades, so this may just be speculation on his part. There also aren't any clear signs that Fox plans on giving up either franchise any time soon. Fox's X-Men films have generally met with success, though its latest Fantastic Four film was poorly received, both critically and financially. The studio will release Deadpool 2 in June 2018, X-Men: Dark Phoenix in November 2018 and has two Fantastic Four movies planned, a Doctor Doom film and a spin-off reportedly focusing on Franklin Richards.