For those of you who read the comics, you’ll know that Magneto doesn’t really feature that prominently in X-Men’s Dark Phoenix storyline.
Magneto could start his cult of mutant followers in this movie, take over Genosha and declare it a mutant homeland, and start Jean Grey’s descent into her Dark Phoenix persona by tempting her with the thought of endless power and mutant solidarity.
Dark Phoenix is set in the 90s, which is the same time that the Acolytes were first introduced in the comics, so it makes some sense.
Which would then invite another storyline into the mix: that of Magneto’s Acolytes and the fictional sovereign nation of Genosha.