While the Marvel Cinematic Universe certainly isn't lacking for characters, not everyone from the Marvel Comics world is able to appear in the franchise. 20th Century Fox still holds the film rights to the X-Men and the Fantastic Four, and there's no indication that Marvel Studios will work out an arrangement to bring either property into the MCU. However, Marvel Comics legend Stan Lee believes that one day Marvel will at least be able to retrieve the X-Men and add the mutants to the universe that began in 2008. As Lee put it:
Sooner or later, they're going to get the rights back to all our characters. 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.
During his recent appearance at Wizard World Nashville (via Comicbook.com), Stan Lee offered a ray of hope for those who've been hoping to see the X-Men incorporated into the MCU and interact with the other Marvel heroes they've spent time with in the comics for decades. It's unclear from his wording whether he's actually privy to inside information or just expressing his wishes for how he hopes the X-Men are handled, but either way, he's all for the property he co-created with Jack Kirby eventually becoming part of the MCU mythology.
It's not impossible that we might one day see cinematic X-Men living in the same world as the Avengers and other MCU characters, but judging by Fox's plans for the mutant franchise, that's not going to happen for a long time. Looking to 2018, the main X-Men film series will continue with X-Men: Dark Phoenix, while spinoffs New Mutants and Deadpool 2 are also scheduled for next year. X-Force, Deadpool 3 and Gambit have also been announced, and producer Simon Kinberg said earlier this year that there are plans for Alpha Flight and Exiles adaptations. As for the Fantastic Four, while the team's 2015 reboot failed critically and commercially, Fox has plans to keep the mythology of Marvel's First Family going through a Doctor Doom movie and reportedly a movie centered on the Richards children, Franklin and Valeria.
Of course, we shouldn't forget that it's not impossible for two studios to work together on a Marvel property. For years Sony was tackling the Spider-Man movies on its own, but following the underwhelming performance of The Amazing Spider-Man 2, the studio joined forces with Marvel to bring the Web-Slinger into the MCU. Granted, Sony is having its cake and eating it too by building a Spider-Man-centric universe through Venom and Silver & Black, but for now, Peter Parker and his supporting cast are resting comfortably in the MCU. Maybe there will one day come a time where Fox is willing to agree to such an arrangement for the X-Men and/or Fantastic Four.