"If it ain't broke, don't fix it."
No matter what they do, they always get great sales (even X-2, which recieved much bad criticism).

My guesses:
Blood Relatives
Alternate Universe (not just another world)

Probably the alternate universe thing. It was stated long ago that the next installment would not be a sequel OR an in-between thing like CoS, so yeah...alternate universe, yay!