I think it will be the leader of the house. one thing i know for sure is that it wont be some lameass servitor, because on the coverart of the HoW dlc, you see some godlike fallen in the background, possibly being the leader of the house of wolves, and if it follows the same trend where crota was in the cover of dark below, and he was the raid boss, then maybe that thing sitting back will be the next.