As popular as the live-action X-Men movies were, the franchise was mostly hit and miss in terms of the quality of each individual film. X-Men: The Animated Series that debuted in 1992 had a much more consistent quality and remains a cult hit to this day. The series' showrunner Eric Lewald weighed in on the challenges of adding the X-Men to the current MCU.
"It's weird. I know that's a challenge. [The MCU has] set up this thing, unprecedented - all these different superheroes that all are successful, and they've managed to give them different tones and different focuses. I'm continually in the awe of this juggernaut that they've set up - I can't use that word because it's a character. I don't know. I don't know how Kevin Feige sleeps at night trying to keep all these balls in the air. But for the X-Men, when people ask me about integrating them in the MCU, I always had the reverse problem. There were too many X-Men and too many villains already in the X-universe and too many connected characters. I found myself cutting half of them out and having stories about three of the X-Men. Just to keep sane and to not have to try to service all of them at once in a 22-minute episode."
Lewald brings up a valid point that the X-Men comics feature more main characters than the entire MCU up to this point. Stuffing so many new characters into an already-overstuffed franchise is going to be a herculean effort. For his part, Lewald remembers his own time on the animated show as being a much more free-form method of creation than the current state of the MCU.
"It's such an immensely huge thing now. There's so many billions of dollars and so many hundreds of thousands of people that are focused on this now. It's so odd to us because this was like a little garage band to us. We just had this stuff dumped on our lap and [they] said, 'Make a show, we'll let you know if it's successful or not.' No real oversight, no micromanaging. Just the show that we and the artists wanted to slap together. It's a whole nother world now."
There is no word yet on how Kevin Feige intends to integrate the X-Men into the MCU, but hints have already started to emerge as to the storytelling devices that are going to be in use. Ant-Man and the Wasp introduced the concept of the quantum realm, which is a microscopic world that has the ability to grant special powers to humans, which could be one way to introduce mutants into the franchise.
Another method could be with the entry of the Multiverse into the MCU, in the upcoming Doctor Strange in the Multiverse of Madness. The alternate worlds that the Sorcerer Supreme visits in his new solo movie are sure to introduce a bunch of new characters, at least some of whom could very possibly be a certain Professor and his school for gifted youngsters. This story originated at The GWW.