The only thing that could compete as far as I am concerned would be a Pokémon MMO. But even that may not work now as Pokémon doesn't have the same appeal it did five years ago. But Nintendo would never go ahead with a Pokémon MMO because of their utter refusal to put anything on any platform that isn't made by Nintendo (that I know of). They could have made so much money off that potential game that the mind boggles at the idea that they never went ahead with it.

I'm not sure if any other MMO would have really worked out too well. I'm sure eventually some company will come up with a master plan and overtake WoW, but I think this will happen at a point when people are simply fed up with the old MMO systems. The new game will need to be groundbreaking to say the least. Imagine the craziest ideas you can think of for an MMO, like artificial intelligence to such levels that the game evolves itself rather than relying on devs. It would need gamechangers on this level. And to be fair, the industry needs this kind of unheard of stuff rather than the many clones out there.