The X-Men. It's only a matter of time before mutants become part of the MCU. When Disney acquired the bulk of Fox's film and TV empire, the film rights to the X-Men and the Fantastic Four returned to Marvel Studios.
Kevin Feige has long insisted Marvel would focus on adapting Black Widow, is a mutant.
Of course, all this doesn't mean mutants themselves have appeared in the MCU just yet. Indeed, Richters' claim Ursa Major is the first mutant is spurious, because he certainly lacked the character's ability to morph into a giant bear in Black Widow. The real question is not how non-marquee X-Men characters and concepts will be woven into the MCU, but rather how mutants will be. The second Eternals trailer may have provided an important clue.
The Hulk's Snap Was A Cosmic Event
Earth may be something of a galactic backwater, but it has become the center of many major cosmic events in the MCU. Several of the Infinity Stones have been hidden on Earth or on Realms bound to this planet, and consequently it became the final stop for Thanos in his quest to collect all the Infinity Stones and erase half the living creatures in the universe. According to Rocket in Avengers: Endgame, "When Thanos snapped his fingers, Earth became ground zero for a power surge of ridiculously cosmic proportions." Five years later, the Avengers gathered the Infinity Stones again — this time using time travel — and the Hulk reversed Thanos' actions by conducting a similar snap. But, remarkably, the second Eternals trailer suggests the power surge associated with the Hulk's snap was even greater. This actually makes perfect sense, because the Hulk's action was a reversal of entropy in the closed system that is the universe — meaning it would naturally be harder to do.
The second Eternals trailer revealed the Hulk's snap has led to what they call the "emergence," an event that appears to be warping the fabric of reality itself, risking the destruction of the entire planet. It's not entirely clear what the "emergence" is; although there's been speculation it is involved with the Deviants or — more likely — will see the return of the Celestials in some way. But that doesn't mean all the energy involved in the Hulk's snap has been caught up in the emergence.
The Eternals Trailer Could Explain Mutants
One shot in the Eternals trailer shows cosmic energy rippling across the entire surface of the Earth, and it's reminiscent of some portrayals of a similar wave of reality-warping power that happened in the comics. event saw Scarlet Witch choose to rewrite reality by deactivating almost all the X-genes on the planet, essentially committing an act of genocide by wiping the entire mutant race out. But several writers realized there was a disturbing question that needed answering, namely: where did all that energy go? They provided a number of answers, with some dormant mutants such as Vulcan appearing and other heavy-hitting super-powered beings created (rather an ironic response, given the near-genocide of the mutant race was driven by editorial believing there were too many mutant characters). Years later, Scarlet Witch's spell was undone by the Phoenix Force in the Avengers Vs. X-Men event, with a similar planet-spanning flare of power.
Marvel could well choose to use a similar idea; to reveal that the flare of cosmic energy associated with the Hulk's snap, or perhaps even this mysterious emergence itself, triggers genetic mutation in a fragment of the Earth's population. This would allow them to introduce X-Men mutants into the MCU at a stroke, and it would certainly make Eternals the most important film in the MCU in Phase 4 to date — one with lasting repercussions on the entire shared universe.
Where Do Mutants Fit Into The MCU Timeline?
How does this theory fit with the MCU timeline? Marvel Studios is hopping around the timeline with impunity. Spider-Man: Far From Home, which is set a year later. Given that's the case, wouldn't mutants have already been mentioned somewhere?
Surprisingly, though, the absence of any references to mutants in films and TV shows set after Eternals is actually easy to explain. As The Falcon & the Winter Soldier made clear, the five-year period between Thanos' snap and the Hulk's saw humanity settle into a new status quo. The return of all who had been lost resulted in another dramatic period of readjustment, with nation-states becoming important again and the old status quo gradually reasserting itself. The emergence of a handful of people with super-powers would easily be missed in the geopolitical chaos following what Marvel calls the "Blip," especially if — as in the comics — the X-gene initially manifests in only a small number of people, with mutants becoming more numerous with the age of time. Thus Eternals' trailer could well have shown the origin of mutants in the MCU, beginning a slow-burn story that will build over time and ultimately lead to the introduction of the MCU's X-Men.
