The X-Men would never fit into Phase 1 of the Marvel Cinematic Universe, and it would be much better if they were introduced in Phase 4 of the MCU. Ever since Patrick Stewart appeared as Professor X in the trailer for Doctor Strange in the Multiverse of Madness. There has been widespread speculation about exactly how the X-Men might finally materialize in the MCU. However, an alternate scene after the Iron Man credits suggests that the X—Men should have been part of the MCU from the beginning- potentially part of the original Avengers team. However, it wasn’t until Phase 4 that there were official mentions of mutants, such as Evan Peter’s Mercury appearance in WandaVision, although this was said to be part of Wanda’s imagination and not Mercury in the X-Men.
The X-Men were originally owned by 21st Century Fox, so Disney and Marvel didn’t have the rights to the X-Men until they acquired 21st Century Fox in 2019. Fox has built its own world for the X-Men, which has led to spin-off films such as New mutants. Throughout the X-Men universe, Charles Xavier’s team has often struggled to protect themselves as mutants and remain hidden until they can live peacefully alongside humans in a future that never materialized for the X-Men.
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Phase 4 significantly expands the MCU, and as a result, the introduction of the Multiverse gives much more opportunities to represent the X-Men. Phase 1 of the MCU was mostly terrestrial, and the only appearances of alien species in the MCU were the Asgardians (whom people knew from old legends and stories) and the Chitauri attack in The Avengers. Although mutants could still be introduced in Phase 1, it is unlikely that they would have been accepted without their own war with humans instead of fighting them against other forces. X-Men films often show mutants threatening humans instead of feeling protected by them, even when they fight on the same side, making their long-awaited appearance in Phase 4 a much more natural choice for introducing X-Men to the MCU.
Thus, the expansion of the “Multiverse” is a great opportunity to introduce mutants into the MCU. New Marvel movies like “The Eternals” show that up to this point, the X-Men could have existed beyond human knowledge, while the Multiverse also shows how many possible alternate timelines and different ways to explain the sudden appearance of the X-Men. Also, after Avengers: Finale, the status of the Avengers and their current mission were less clear. Despite the fact that the initiative is still running in the MCU, there is currently no clear team and a particular shortage of Earth Avengers. Many of them are fighting in other areas of the galaxy, such as Captain Marvel (Brie Larson), or are currently stuck solving problems and threats in their lives, like Wanda Maximoff (Elizabeth Olsen).
So this lack of stability for the Earth Avengers team provides a great opportunity for Marvel to introduce the X-Men and rebuild the Avengers by making them the focus of the MCU. Spider-Man is also a mutated human, which makes Phases 4 and 5 an ideal time to introduce mutants, as science is now already part of the MCU. The X—Men are people with the mutated X gene, so Earth is their home, and they will probably fight for it, which makes them an ideal option for a new Earth team. Trying to fit them into the limited Phase 1 of the Marvel Cinematic Universe would be problematic, but the introduction of the X-Men in Phase 4 represents a great opportunity to integrate these iconic mutants into the MCU.