Overtaking the "#1 MMO spot" is like taking over the best soft drink spot. It's all flavor and no matter what you do people will love one over the other and the rest will flock to the most popular.
Regardless of that, I doubt anything in the next 5 or so years will shake the MMORPG foundations to the point where Blizzard can no longer stand on. Their roots are firmly in place, if not just because of their past games and names they use. The genre would either have to fan out naturally or something terrible must happen much worse than what people considered "the worst expansion ever" (Cataclysm).