I think the vex will be next. Since you have killed Oryx, the God of the hive, and the vex have been trying to imitate the hive to become immortal (according to the grimoire), the vex would logically take the opportunity of seizing the drednaught and some other stuff that they can make a story out of. I think this will be the next expansion. I think 2k16 Destiny will logically focus on House of Kings. As another guy pointed out but I will go further in depth. Heart of the Garden, Crota, Skolas, Oryx, Vex seize the opportunity, Cabal reinforcements arrive House of Kings finally finish planning whatever it is they are planning,..........