James McAvoy is playing a super-powered villain in Glass, but the actor is perhaps best known for his role as Charles Xavier in the X-Men franchise. With the closure of the Disney-Fox deal looming, the X-Men will soon be making their way back to Marvel, at which point it is a matter of if, not when, they join the MCU. To that point, James McAvoy isn't so sure if the X-Men will work in the MCU, as he explained:
I don't know if the X-Men could go into the Marvel universe, I'm not sure. Maybe they could? But I think what's different about the Avengers universe anyway is you've only got a couple of superheroes in the world. There's a good amount, but there's like a couple of football teams' worth, you know what I mean? Whereas in the X-Men world you're potentially saying there are hundreds of thousands, maybe millions of [superheroes] and the social implication of that is different.
Professor X is known for his smarts and James McAvoy brings up a cogent point here about the different dynamics between the world in the X-Men films and that of the Marvel Cinematic Universe. We tend to think of the MCU, in its scope and tenure, as having tons of heroes, but as James McAvoy told Yahoo Movies UK, it is really only a couple of football teams' worth of superpowered characters.
Contrast this with the world of the X-Men, where superpowers aren't afforded by wealth, experimentation, accidents, technology or godhood, but by genetics. They may not all be superheroes, but the world of the X-Men is one in which superpowers are prevalent as the next stage of human evolution. That drastically changes the playing field.
As James McAvoy said, the X-Men situation has different social implications. The Sokovia Accords came about because of the actions of a handful of powered people; there are millions of powered people in the world of the X-Men, and that has to make a difference. James McAvoy isn't saying that the X-Men won't work in the MCU, it's simply that he isn't sure given the nature of the stories. And to be fair to him, we won't know that it will work either until we see it.
Obviously the X-Men, the Avengers and countless other Marvel characters work together in the comics, so it can be done. But those disparate elements grew together organically. While he doesn't state it exactly, James McAvoy's uncertainty raises one of the key issues we have often wondered about concerning the X-Men joining the MCU. It's a question of compatibility.
Integrating the X-Men films and actors from the current Fox franchise would be exceptionally difficult and all signs are pointing towards a reboot of the characters within the MCU. But compatibility concerns still remain, specifically regarding the timeline. Peter Parker could be bitten at any point and inserted into the MCU, but mutants are an evolution that requires buildup.
Will mutantkind just be arising when they appear in the MCU? X-Men stories are about segregation and persecution of those that are different, and that allegory doesn't work as well if mutants are just becoming a thing in a world where people look up to Captain America and the Hulk.
Conversely, if mutants and the X-Men have been around all along, why haven't we seen or heard about them? Despite Kevin Feige saying that he hasn't thought about it, you have to think that Marvel has some ideas about how to make all of this work. Perhaps some post-Avengers: Endgame, time/reality shenanigans can be used. Whatever it is, the answer isn't blatantly obvious and that's what James McAvoy highlighted.