Ok, I would not normally say anything about the rumors spread by mmo-champion and others however I want to say one thing. I just finished the new World of Warcraft Comic #22, and all I have to say is if there is going to be a new Guardian it is not going to be Thrall. Thrall may be involved with creation of a new guardian even though he sent along one of his other shaman to help.
I will not say specifically who I think is going to be the next Guardian. All I will say is that I think it became almost a hereditary position. Given some of the new lore they are creating with the various comic series. Also if we defeat Arthas in 3.3 then the next old gods part will most likely come out in 3.4 to set the scene for 4.0.