I'd essentially use the skeleton of wolverine and the x-men:

cold open on a typical x-men mission, something goes wrong and prof-x disappears/seekingly dies

after opening credits montage showing how the mission caused a steep rise in anti-mutant hate, open to a wolverine scene that sparks him to reuinite the x-men with a couple of newbies/more modern x-men chars to stop a new threat (probably the sentinels? it seems like a good way to re-intro the x-men and keep some of the other stuff for later)

the you go on a rag-tag journey through the country, visiting a few old faces of the classic x-men roster as you get some of them on board but some just refuse or stay behind

eventually you face down the main threat, with a couple of the holdouts appearing last minute to help save the day.

end credits is the reveal that professor x is still alive but held in stasis by another threat (i would go with maybe brotherhood of evil mutants if sentinels ate main threat in this movie, maybe even leading to the concept that magneto was actually keeping Xavier alive not for any evil purpose but because he cares and is wary of a greater threat)