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I can't think of anymore. List any you think might also join the list.
There are many other popular IP's (Star Wars, Warhammer, Lord of the Rings, Narnia, Final Fantasy etc.) but none come close to how popular those three are in todays society.
But Nintendo refuses to make an MMO, and I can't see a Harry Potter MMO working, so in other words, only Blizzard will dethrone WoW (either with a new title, or just by slowly destorying WoW unintentionally, which would still take years.)
Edit; Just to clarify, I'm not saying only a game with a well known IP can dethrone WoW, I think a very well made game that plays well could easily do it. But in this thread I'm just asking what existing IP's you think could dethrone WoW.
But let's be honest, a good game with a well known IP will do much better than a good game with a new IP.
Look at Okami, probably the best PS2 game ever and one of the most critically acclaimed, and yet hardly anyone has heard of it. (Ico I'm looking at you too!)