Sebastian Shaw was not the only powerful member of the Hellfire Club, and even though they were seemingly wiped out in First Class, theres no reason to believe there couldnt be other members lurking around in the shadows. Take Harry Leland for instance, the wealthy and powerful Black Bishop of the inner circle of their cabal, with the power to increase the mass of those around him. Deliberately modelled on Orson Welles, Leland seems like the furthest thing from a physical threat. He can barely hold his own one-on-one, but its that unassuming nature that makes him interesting. People responded to the powerless William Stryker because of his methods and worldview, and Harry Leland could accomplish that same feat with his elitist views and manipulative ways. Most importantly, Leland would allow the film to tie back into First Class, establishing a through line in the series and giving him a very clear revenge-motivated mission. Going forward, his presence could also be crucial in redoing the Dark Phoenix Saga, something all fans want to see happen.