I know it's not possible (and I do want to read the whole thread and will eventually) but I'd say a Pokemon MMO. Done right by Nintendo (and probably paired up with another Western company for consulting) and I think Nintendo could do it. But it wouldn't dethrone WoW by competing with it (I think the target audience is too different) but by having more subs then it. But said MMO will never happen. Nintendo has said so (they don't even like Pokemon games for the consoles so much) and if they did... I dunno if it would turn out well without a Western company's help. *looks at Dragon Quest 10*
But besides that, WoW will die to old age and subscribers bleeding off slowly as is. Even then... WoW wouldn't just "die" but fade away from the limelight.