Doctor Strange in the Multiverse of Madness is bringing in the Illuminati, which seems to be led by Professor X. We clearly heard Patrick Stewart’s voice in the trailer, even though the actor suggests otherwise. But is this Professor X the same that we saw in all those X-Men movies from Fox? Given that Kevin Feige recently brought back the past two Spider-Men and their franchise villains, one could think that he may be bringing the X-Men back too (among others). But I don’t think the same logic could be applied to Fox’s X-Men for several reasons. I believe that Professor X in Doctor Strange 2 has to be a variant that looks like Fox’s Charles Xavier and nothing more.
Incorporating the other 3 Spider-Man universes into MCU’s multiverse was easy as they didn’t use time travel in their stories. So, the MCU could actually fit them in their multiverse without creating any sort of plot hole. But if Fox’s X-Men are acknowledged, then a major plot hole would be created since those movies dealt with time travel very differently.
MCU’s Time Travel
As Endgame thoroughly established, time travel works differently in the MCU compared to most films/franchises. There are three types of time travel rules that are followed in movies and TV shows. The first is the conventional “Mutable time travel” which we’ve seen in films like Frequency, Back to the Future, and Looper. Here, changing the past would change the future. A person or an entire set of events could get erased from existence. People could even manifest new memories based on their changed past.
The second kind is “Immutable time travel” where a person’s fate is destined. Whatever has happened to you will always happen even if you try to change things. In fact, your time travel shenanigans might actually be a cause for your destiny. We witnessed this in Harry Potter and Netflix’s Dark.
Then there’s the third kind, which we’re simply going to call the “Branch Theory.” The MCU follows this multiple timeline theory. Apparently, even Dragon Ball Z followed it. Here, changing the past does not change the future. It would just create a new branched reality. And that reality becomes a part of the multiverse.
Fox’s Time Travel
So judging by these time travel rules, both Deadpool 2 and X-Men: Days of Future Past proved that they belong to the first category that is “Mutable time travel.” In Deadpool 2, we got proof that by stopping Russell, Deadpool saved the demise of Cable’s family. It was showcased by the changing color of Cable’s daughter’s Teddy bear on screen.
In DoFP, as soon as the younger Professor X convinced Mystique, the post-apocalyptic future events got erased and the whole timeline was reset. These are the kinds of plot elements that were proven to be completely bogus in Endgame. Smart Hulk spent a considerable amount of screen time denying this logic as the events that have already happened cannot be changed no matter what we do.
So, if Marvel were to bring in Professor X from Fox’s universe, or even Deadpool from Deadpool 2, then they’d be automatically acknowledging the flawed time travel rules of that franchise. It would make no sense for time travel to work differently in the Fox universe compared to the MCU if both these universes are supposed to be a part of the same multiverse. A gigantic plot hole will be created and the MCU will probably dig a big hole for itself.
The Fix – Professor X in Doctor Strange 2 is a Variant
Nostalgia worked big time for Spider-Man: No Way Home as we got to see the same characters from the past franchises interacting with the present ones. It would be amazing if Marvel could do the same with Fox’s X-Men franchise. But the extensive use of different time travel rules in those films prohibit that. Maybe the fans would be willing to turn a blind eye towards this time travel flaw, but Marvel shouldn’t. They should not create a plot hole willingly when they can totally drive the nostalgic feeling in another way.
The X-Men version of Professor X died. Hugh Jackman’s Logan died. We should let those versions stay dead. Meanwhile, the MCU could just use a Professor X from a different universe, (let’s say the Illuminati-verse). He just looks like the one from Fox’s franchise but is a different character. In this universe, he met with the Avengers and several other heroes who weren’t in the Fox Universe. The fact that this Variant Charles Xavier would look like the Fox version would be enough to make us reminisce about the past.
The same variant logic could be applied to Hugh Jackman’s Wolverine if he ever shows up in the MCU through the multiverse (maybe in Secret Wars). He could just be a Variant Wolverine. And besides, Fox’s X-Men movies had a lot of baggage. There were a few good ones but there were many bad ones too. So, it’d be better to stay away from them.
To conclude, I’d say that Fox’s X-Men universe is off-limits for MCU. Marvel shouldn’t make it MCU canon unless there’s a way to fix the time travel issue. Rest, they could make any past Marvel movie/franchise they like canon (even Howard the Duck :P).