Okay, we’ve done threads on your personal favorite choices for Justice League members, and Mighty Avenger members, so I guess this is the logical next step.
First, I’d start with the Beast, because he’s one of my all-time favorite characters. He’d take over Prof. X’s position as headmaster of the school and behind-the-scenes leader of the team.
Kitty Pryde - My second favorite X-person. She’s smart & experienced enough to serve as the team’s field leader.
Sunspot - We need a superstrong guy, and he’s was always itching for a promotion to the X-Men back in the “New Mutant” days. It’d be an interesting contrast (to his former arrogant persona) to see him get the job, and then not like it.
Magma - Just 'cus she looks way cool in her molten lava state.
Skin - Stretchy elastic guys always come in handy when you least expect it, and he’s cooler than Warlock, so sign him up.
Feral - Every team needs a good brawler, and she fits the bill. The last time I paid attention to Marvel continuity, she had become a villain, but that is a mere minor change in the world of the X-Men. She’d be a (somewhat) reformed bad girl.
Legion - Because the team needs a psi, and it just wouldn’t be the X-Men if there wasn’t a loose cannon on the team.
Nightcrawler - Teleporting, near invisibility in shadows, wall-crawling - all tactically useful. He’d be considered the ‘old-timer’ by his younger compatriots, and get chided for always talking about the ‘old days’.
As you might have gathered, I like the idea of clearing away the old guard and giving promotions to some of the younger X-hanger-ons. IMO, some of the longtime characters such as Cyclops & Wolverine seem fairly stale now. I’d like to see a new team (akin to the “new” X-Men of UXM #94) step up to the plate.