I'm with you, them fighting each other is old now. Magneto doesn't even have to be center-stage in future films; I'd settle for him being a background character with an established haven for mutants, like the Genova concept Dark Phoenix seems to be going with. I just think some form of him needs to be present because his attitude and philosophy is given a lot of conviction given the fact that he's a Holocaust survivor. I just don't think any other villainous character in his camp has that kind of credit or relatability.
Xavier, they could do without. We don't need him constantly so much as we need his followers trying to achieve his dream without him. Alternatively, have Cyclops lead the X-men and try to choose his own, middling path between the philosophies of Xavier and Magneto.