Technically, the Marvel Cinematic Universe has already told multiple X-Men stories. Disney had acquired FOX years before Doctor Strange in the Multiverse of Madness premiered. If Marvel Studios wanted to, it could make the story of the two X-Men trilogies as well as the several spin-offs canon even if they make it so that they'd take place in an alternate reality or universe. This is a problem it will eventually have to take care of with Ryan Reynolds and Deadpool set to make their MCU debut with Deadpool 3.
Having said that, Avengers: Endgame's directors, Joe and Anthony Russo, might have just hinted at what it would take to make them return to the MCU.
The Russo brothers are the most financially successful MCU directors. The two made their MCU debut with 2014's Captain America: The Winter Soldier. They later came back to direct 2016's Captain America: Civil War. More importantly, the duo was tasked with wrapping up the decade-long story of the MCU at the time with 2018's Avengers: Infinity War and 2019's Avengers: Endgame. Their work on the latter two films would cement them as legends in Hollywood as the pair of films combined to gross nearly $5 billion at the global box office.
