When Marvel Entertainment faced bankruptcy in the '90s, they were forced to sell the film rights to many of their most iconic superheroes. They've since regained some of them but the X-Men and Fantastic Four remain off limits to them. 20th Century Fox own both franchises and while Marvel Studios President Kevin Feige has said in the past that they could enter the MCU, it seems unlikely.
Well, not according to Stan Lee! During a recent appearance at Wizard World Nashville, the comic book creator revealed that he believes Marvel is currently working on regaining the rights to those.
"Sooner or later, they’re going to get the rights back to all our characters," Lee promises. "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." It's important to not that Lee has no involvement in Marvel's day to day operations so this is probably just speculation.
The lines have blurred in recent years, though, with Marvel using characters like the Skrulls and Ego the Living Planet and Deadpool featuring a Helicarrier. Whether or not we'll ever see the X-Men and Fantastic Four standing alongside the Avengers remains to be seen but stranger things have happened.