I find it very telling that the lore has been so damaged by this xpac, that there isn't even much speculation about what happened to Gul'dan. Its like the fanbase feels like "Oh, well, he'll just show up later to be fodder for us in some dungeon or raid later on." I mean, we just killed Archimonde, how is saving Gul'dan for later supposed to strike fear (or even interest) in us?