Found this in another website, take it with a grain of salt
Fall is a prelude to a much larger event coming next year called End of X.
Death of X is about a group of mysterious assassins targeting specific X-Men for death, some big names die and the culprits turn out to be a group of people who were being controlled by an unknown intelligence from the future.
End of X comes out next summer. Carol Danvers teams up with Timegazer (he's the new inhuman who can peer into the future from Civil War II; also Carol "wins" Civil War II and the whole thing is much lower stakes and less edgy than Civil War I) to discover a new threat looming, some kind of horrible "storm" that threatens to collapse the timeline.
The "storm" can only be resolved by removing all the mutants from the universe, which sparks big time chaos about how to resolve the issue.
I don't know the exact plot details (doubt anyone does because this stuff isn't usually ultra-thought out beforehand except for the broad strokes plot), but it ends with all the mutants being shifted over to another timeline of their own free will. The 616 goes on without them, and there will still be X-Men books but they'll all take place within their own separate continuity to ensure maximum MCU synergy within the "main" universe.
They've been looking to do this for a while. Folding the ultimate and 616 universes together was to make room for a potential "X universe" down the line, to avoid too many alternate universes running in parallel. Some specific mutants *might* stick around in the 616, but I don't know which.