Yeah I think when it comes to which X-men film is the best, a lot fans are split between FC and X2.
I don't think having the film focused on FC cast would harm the film. FC cast is the future, while the OT cast will inject some nostalgia into the film, if they used appropriately and have less screentime, I think it would still satisfy fans.
If the film started with Magneto saving Charles taking him to the X-Men's hideout. Having the two talk about the old times and what not, basically an emotional charged scene, where they go through the mistakes that they made. All of a sudden the sentinels arrive. Magneto uses everything he has to protect the X-Men.
Basically a scene like this, that would lead to his death.
I'd say that would be more then satisfying, even if he only gets like 20min of screentime.