Bringing the X-Men in as the Avengers’ foes in “Avengers: Secret Wars” would be a smart move in more than one way. First of all, the MCU has spent years and years establishing its assorted Avengers, and in order to adapt an event of the magnitude of “Secret Wars,” the team needs an opposing force the audience is invested in. This is very hard to pull off with freshly introduced villains, and let’s face it, 2027 is such a long way away that alternate-universe variants of existing heroes will be old hat by that point. The X-Men would be a perfect solution to this issue since they come with pre-built name recognition and a robust cinematic history of their own. What little introductions are needed won’t be a problem either, thanks to Monica — a newcomer in the X-Men universe who can easily act as a viewpoint character.
This would also be a fantastic way to underline the MCU X-Men’s underdog nature. For most of their Marvel history, the mutants’ whole deal has been their outcast status, and having them battle a celebrity superteam like the Avengers would be a perfect way to put them in this position.
The two teams also have a history of butting heads in the comics, perhaps most notoriously in the 2012 “Avengers vs. X-Men” event. Such a battle would also give the MCU a perfect excuse for a “Captain America: Civil War”-style situation where heroes fight heroes in an epic showdown. Now that the ending of “The Marvels” has opened the door for this opportunity, it will be interesting to see where “Avengers: Secret Wars” takes things.