Tirion is the 'main hero' of WotLK all the way from the start to the end. Then after he setlles back down in Hearthglen, his story abruptly ceases. Not a single appearance from him is made since. He's absent throughout the entire Deathwing drama, and doesn't bother to at least show up at the council meeting regarding the Sha threat. Apparently, he doesn't even come when Garrosh merges with one of them.
So, is that the end of his tale? I'd sure like to see him, as well as a few other characters (say, Medivh) show up to fight Sargeras and the return of the Burning Legion.