It's no secret that after Secret Wars Marvel plans to delve into the Mutant Saga. However, we don't know what "Avenger's Level" or equivalent movies they have planned. I've seen people want Avengers vs. X-Men, but it seems like at least some of that will be covered in Avenger's Doomsday and Secret Wars. They also could adapt the Genosha or Krakoa attacks, but I feel they will do a large scale galaxy wide conflict involving the Avengers.

Thus, I think we could potentially see House of M or the Dark Phoenix. House of M where Scarlet Witch infamously says "No More Mutants" almost wiping the mutant gene out of existence. I think this could happen as although parts were adapted in Multiverse of Madness, Elizabeth Olsen (Wanda's Actress) has spoken publicly many times about how she wants to say that line and do that arc. With the X-Men coming, it's prime opportunity.

The Dark Phoenix saga, where Dark Phoenix Jean becomes an Cosmic Avengers level threat is such an emotionally strong arc. It's one of the most popular arcs in Marvel comics. We have seen some iteration of Dark Phoniex adapted in pretty much every X-Men portrayal Jean is in. I'm leaning more towards Dark Phoenix given that Marvel has expressed it will dive more into space and there are rumors we may see the Shi'ar.

What do you want the climax of the MCU's mutant Saga to be?
What's a Better Climax for the MCU's Mutant Saga? House of M v. Dark Phoenix