It would be nice if this movie could possibly capitalize on the Walking Dead craze with a set up for Necrosha and also boost popularity for Legion TV show. I think it might be interesting to play this as a two part type movie with Selene being the villain. And while we don't know for sure everything about FX's Legion if the created some synergy there it might make for good run.
Maybe movies can deal with the recruitment on one side and a bit of horror type mystery on the other. You could have maybe the disappearance of young female mutants and suspicion of serial killers, sex traffickers, and cults. Ultimately it's the black priestess cult headed by Selene either draining the life-force of young women while attempting to reach godhood. Moira is on to this and gets Xavier to recruit young mutants. This is where the recruitment and training comes in and we get to see the origin stories. With the recruitment of Amara we get the team going head up with Selene. Though they win in the initial encounter they have to go after Amara and Selene escapes going to the school and taking out Xavier. She replaced him with Legion who is now under her influence after partially draining him. She's fluencing him and causing him to alter reality projecting the illusion that he's Xavier. We get the Demon Bear Saga from this as its a real life illusion projected by Legion-Xavier to wipe out Dani Moonstar because she senses something wrong . Xavier-Legion wants the team to recruit others, but this is Selene of course controlling things, when the team refuses the true nature is revealed. Eventually all this leads to Necrosha where we are able to appease the zombie genre (Walking Dead fans) with Selene attempting to ascend to Godhood.
I don't know I'm definitely not a screenwriter lol so this all probably sounds like gibberish. I hope the movie incorporates some of these elements though and stays relatively grounded instead of getting really wacky with extraterrestrials and all