For Iceman fan casting, I always go back and forth between Lucas Hedges and Harry Styles—two very different performances, but I think either would be exciting choices.
I’d like to see the original 5 in an opening X-Men film, but maybe they’re not intact as a team anymore with new mutants having already moved into the mansion (Storm, for example). Like, Krakoa or something similar has already occurred, and we as an audience are being introduced to the X-Men in the midst of their ‘second class’—you can even drive it home w/ a Polaroid of the team in their classic, comic-accurate uniforms.
Warren is gone, having left the mansion to run his father’s company. He's back to hiding his wings in a harness under his suit. And Beast left to work with Moira MacTaggart on Muir Island. Both can be cameos/supporting roles, but the primary team/focus would be Scott, Jean, and Iceman at the mansion joined by Storm, Forge, and Jubilee (who is moving-in at the film’s start after the recent emergence of her powers--so the audience gets further exposition as she’s introduced to new elements).
And, for story, I do think that the MCU’s X-Men can jump straight into a confrontation with a new iteration of the Brotherhood, and comic-accurate Sentinels. The general audience can easily forgive some exposition early in the film to set up the idea of mutants in the MCU—something simple enough along the lines of reports of individuals seen exhibiting strange powers or physical appearances. Real click-bait/National Enquirer stuff.
Open the film, or introduce it early in the script: A montage of cell phone / YouTube footage of potential mutant sightings being discussed on a Real Time with Bill Mahr-type talk show hosted by Christine Everhart (from Iron Man & Iron Man 2) with special panelists: DailyBugle.net Editor-in-Chief J. Jonah Jameson, New York senatorial candidate Robert Kelly, & renowned geneticist Charles Xavier. Kelly supports stricter monitoring for these so-called ‘mutants’, using the public’s paranoia after the Sokovia Accords and The Blip, and every world-ending MCU event in all fairness, to his advantage.
Mutants aren’t public knowledge yet, but the Brotherhood of Mutants (Avalanche, Blob, Toad, & Rogue, led by Destiny) begin targeting Kelly campaign headquarters, making it tougher to keep the lid on their existence (Destiny sees a dark path for mutants with Kelly in office). Xavier deploys his X-Men to stop the violence & recruit the Brotherhood to their peaceful cause of coexistence, but an eventual public brawl at a political rally outs the existence of mutants to the world, and the public turns to panic.
While the X-Men and Brotherhood clash, Sentinels are deployed to address the threat to homeland security as Project Wideawake is green-lit, lead by the enigmatic Tanya Trask who's grandfather, Bolivar, worked for the Strategic Scientific Reserve in WWII—not previously seen in other MCU films because he was working on the Pacific front. Perhaps this time period/setting can be revisited in future MCU films, exploring Bolivar clashing with Namor, given the latter’s history as the first mutant and his WWII origins as a Defender). Destiny’s actions to better the world for mutants have damned them all to the dark path she foretold.
End the film with Professor Xavier addressing the press for the first time with the X-Men (now joined by Rogue who switched sides during the film) as mutantkind, or maybe with Jubilee answering a knock on the mansion’s front door to find Nick Fury standing there looking for answers in a scene meant to end the film with a laugh—can’t figure which I want. Maybe the latter can be a secondary credits scene.
In a post-credit, reveal Kelly’s close personal aide to be Mystique (comic-accurate costume, please) and Destiny’s partner.
It’s a lot, but I think pretty doable for a Marvel feature based on what we’ve seen.
