I reckon at the end of the game the final boss will be.... ourselves!!! We will have become so powerful that we are a danger to the world itself and have to fight our souls for entry into the next plane of existence..... it would of course be a 40 man raid based entierly on vehicle combat in the style of ToC (awesome!!! )
Once you have looted your "saved soul" from the final boss the game ends and you can spend the next 2 months re-adjusting to life without WoW.