While there are no X-Men properties coming to the Marvel Cinematic Universe in Phase Four, Phase Five appears prepped and ready to bring the mutants back into the fold in a huge way. Mutants and the X-Men remain among the most noteworthy characters in the Marvel Universe. For a time in the '90s and '00s, the X-Men far surpassed the Avengers in popularity. Now, however, the puzzle becomes how do the X-Men fit into the massively successful MCU?
There are several ways the mutants might emerge in the MCU, with some being surprisingly mundane and others being alarming gargantuan. But how do the mutants and X-Men arrive? Here are a few possible ways given what we know about Phases Four and Five of the MCU.
They Just Show Up
How did Wakanda, sorcerers, the Eternals, the Kree and Celestials arrive in the MCU? They just showed up. While this might be the most underwhelming answer, it is the most likely as well. With the confirmation of Blade arriving in the MCU, it's clear that vampires will have been in continuity the whole while. How did no one notice them? Well, they lived in secret.
The same can be true for mutants. Perhaps they lived underground, hiding their gifts this whole while. The Xavier School for the Gifted is a place where mutants can foster their abilities in a safe place. No doubt Charles would keep it secret somehow. Perhaps he uses his telepathy to erase the memories of all who see it. Perhaps the X-Men are very good at being stealthy.
The fact is mutants might already exist in the MCU. However, perhaps the mutant gene is well hidden, with some other event triggering the mutant gene awakening in the larger population.
Spider-Man: Far From Home teased that the Blip resulted in holes being formed in the multiverse. While this turned out to be Mysterio's deception, the idea of the Blip or Snap causing some glitch in society's genetic composition seems not too far-fetched. Perhaps the bombardment of radiation from the Infinity Stones activated the mutant gene, creating the mutants.
After all, consider how the Mind Stone was enough to enhance the Maximov twins. With all six at once, being activated again and again, the side effects might've already had a drastic impact on human society. Much like fallout from a nuclear explosion, however, the effects might take awhile to manifest.
If so, the mutants are already among society. In some regards, this is neater and easier. It can be true that mutants have always existed, but now are just more common thanks to all the stones' activity.
House of M
However, then there is the potential of House of M. Wanda Maximov will be taking a more prominent role in Phase Four, with her appearing in both WandaVision and Doctor Strange in the Multiverse of Madness. Both will further delve into her grief following the death of Vision in Avengers: Infinity War while further developing her abilities.
Many have speculated that Wanda's grief and increasing control of her abilities will result in a House of M situation, where Wanda rewrites reality in some fashion. This rewriting of reality could restore life to her father, who, in the comics for quite some time, was Magneto.
Whatever the case, Wanda is being set up for something. Phase Three and Four appear to be pushing her farther to some end, but what? If it is House of M, she might just create mutants.
Celestials Did It?
A fan theory posted some time ago might have cracked the mystery right away: the Celestials made mutants. This theory ties the origins of mutantkind to the creation of the Eternals. While the theory isn't completely sound and has some huge logical inconsistencies, some variation of this theory may end up being proven true.
While the Eternals are a race of immortals, it is possible that the Celestials' experimentation could create the mutant gene thousands of years prior, which gave rise to mutants such as Apocalypse. Perhaps the gene remained latent until some cataclysm triggered the genetic mutations, such as, say, the Infinity Stones activating again and again on Earth?
Tying mutants to the Eternals might offer further continuity between the earthbound affairs of the MCU and the greater cosmic activity of the universe.
Doctor Strange in the Multiverse of Madness appears to confirm that, yes, multiverses exist and influence the activity of one another. It is possible mutants already exist in the MCU and that, somehow, Strange's meddling in reality will cause two universes to blend together.
If this is the case, this might result in some drastic alterations in continuity. Two universes bleeding together might alter history entirely. It might also leave several mutants, now interdimensional travelers, displaced and without a home, creating a potential immigration allegory. Perhaps they can find asylum in the home of another group of refugees: New Asgard.