The X-Men are being rebuilt. Ever since Disney took on the rights, the open question has been how a franchise that lived at 20th Century Fox, entirely apart from the Marvel Cinematic Universe, gets folded into it. Spider-Man: Brand New Day answered part of that already.
Jean Grey went first
Sadie Sink played Jean Grey in that film, a powerful telepath and one of the central figures on the team. She spends most of the running time as the antagonist, then saves Spider-Man and leaves on her own, presumably to go and find others like her.
The cast, announced at D23
Disney used D23 on August 14 to fill in the rest of the roster. Kit Connor, of Heartstopper, plays Scott Summers. Samara Weaving is Emma Frost. Christopher Abbott takes Professor X. Maya Boyd plays Storm. Adam Driver is Mister Sinister. Inde Navarette is Rogue. Sink carries on as Jean Grey.
Sink's own line, offered before the names went public, was that a good cast was coming together and that people would be excited once the whole thing was announced.
Who is making it
Jake Schreier, who directed Thunderbolts, has the film. The script comes from Lee Sung Jin, the creator of Beef, and Joanna Calo, co-creator of The Bear, which is an unusual pair of writers to aim at a superhero picture and probably the most interesting thing in the entire announcement.
What it is about
The plot is being kept quiet, and there are more possible directions than any one film could use. The premise underneath the X-Men has never changed: people begin mutating spontaneously and acquire powers, some of them using what they get for good and some strictly for themselves.
The real conflict has never been between mutants. It comes from everybody else, from a public that either accepts them or would prefer them controlled and is not especially concerned about what that takes. Professor Xavier, a telepath, has spent years trying to integrate mutants into ordinary society, teaching students at his academy while leading a team drawn from the most powerful of them.
What to watch before it
Avengers: Doomsday arrives on December 18 and is the obvious place for more to be revealed, given how the studio has been releasing this material a piece at a time.