Nostalgia is the hollow remnants of memories long gone.
-Kaito Kumon (Kamen Rider Baron)
I think that if we were to face a titan as a nemesis in the game, that would mark the end of the game. I just don't see, based on all the information I've encountered on the titans, how we would ever be able to face them and win. When/IF Sargeras ever does make it into the game, I don't see how we as the heroes will actually be able to defeat him. Broxigar is dead and can't pin-prick him again!
Edit: Typos, etc.
I like your thought process OP, but remember: Halls of Lightning and Halls of Stone are still part of the Ulduar complex. Its absolutely huge and covers pretty much all of northern Storm Peaks. Even the Engine of the Makers is part of Ulduar as I understand it.