I don't know why everyone wants Magneto to bite the dust or leave for another villain to come center stage in X4 (crossing fingers). Magneto has been greatly explored and is just an overall great character. I love the ideas and positon on mutancy/homo sapiens. To me, it is simply fascinating and IMO much more believable and achievable than Xavier's dream of coexistence. Apocalypse and Sinister are for mutan supremacy but they go about it differently than Magneto would. Also they have really no history with any of the other characters, so there's no emotional connection. The Hellfire Club is only seeking world domination by political and econmonic ways, making them quite generic and bland.
Magneto should be in every X-Men film. Yes, other villains can be introduced, yet always have Magneto in their.