Everyone is wondering how the X-Men are ever going to fit into the existing MCU, but one Avengers: Endgame theory offers a logical way that it could happen. During Infinity War, Thanos is collecting the last of the stones that he needs to snap half of the universe out of existence. After killing Vision for the mind stone, Thanos uses it to fulfill his promise. This is an enormous amount of power, more than any being has had before him.
After Infinity War, the heroes are trying to hunt down Thanos so that they can find him, defeat him, and use the infinity stones to bring everyone who died in the snap back from being dusted. The search for Thanos continues until Nebula recalls where he always said he would go after he performed the snap: The Garden. After learning this, Rocket Raccoon says: "When Thanos snapped his fingers, Earth became ground zero for a power surge of ridiculously cosmic proportions. Nobody's seen anything like it, until two days ago, on this planet." The Avengers then go after Thanos and learn that he's used the stones to facilitate their own destruction, which is what must have given off the second enormous power surge.
After the time heist, the Avengers collect all the stones and use The Hulk to snap everyone back into existence, ostensibly setting off another giant power surge, this time on Earth. Then, when Thanos comes back and threatens to eliminate 100% of life in the universe, the Avengers bring their own infinity gauntlet into the mix to collect all six stones and defeat Thanos's army, snapping him and his minions out of existence. That's three times that the stones were used on Earth, with the fourth being when Thanos snapped the stones away from The Garden. It stands to reason that each of these three power surges, coming from Earth, set off a huge chain reaction of events. One theory (via Reddit) states that this could help explain the introduction of mutants into the MCU.
How The Snaps Could've Led To Mutants In The MCU
According to the theory, the massive amounts of energy released when Thanos, Hulk, and Tony snapped created waves across the world, activating the X-Gene in people. Then, as the theory states, over the following several years, the X-Genes will start to become active, which will lead to mutants becoming part of the population. This is a clean and clear way to tie the previous events of the MCU into the next phase and would perfectly explain why mutants have been absent up to this point.
By this logic, the snaps themselves will not grant people superhuman powers but will activate the gene to bring out what was already there, like what happened with the Scarlet Witch. WandaVision confirms that Wanda did not get her powers from the mind stone. Rather, the stone activated abilities she already had. This suggests that not only do the stones have that power but that Wanda's a mutant as well. It's possible that all the other potential mutants were already out that, waiting to be activated by something like the snaps, and that this is how the X-Men will fit into the existing MCU.
