Age of Conan? Nope. AoC will be a niche game. Heavy graphics requirements, its mature rating and its attempt to have a PvP-focused game while still appealing to the raid AND the 'casual' crowd at the same time will keep its population low.
Warhammer Online? Nope, though I expect WAR to be in 2nd place in number of subscribers for a Western MMO unless they just really drop the ball or lose focus on what their game should be.
Some other game? Nope
What will kill WoW is WoW itself. It's inevitable, and expected. The game gets stale, players get bored, move on to other activities (be it MMOs, other games or real life events), sever populations begin to fall, servers get merged, devs start working on other games, etc etc.
WoW will follow in the footsteps of other successful MMOs and have a long, long slow descent into the twilight realm, joining its forefathers such as Ultima Online and Everquest in semi-obscurity.
So all the talk about such-and-such game being a WoW-killer is just dumb and a waste of energy. Nothing short of a nuclear holocaust is going to cause World of Warcraft to suddenly lose millions of subscribers.