First off, we haven't seen the Kell of the House of Devils, Kings, or Exile. House of Exile Kell could be a Fallen-Hive mutation. Then there are the four hive gods we won't be killing in the next DLC. Xol, Yul, Eir, Ur, all either equal or higher in power than Oryx. And all with their own Crotas. The vex could have multiple vaults of glass. There could be another Atheon, another Black Heart. And there are lots of Minds that the vex could have. The Cabal are a giant military, but they could be factions of a larger empire. Their emperor could be a target. Then there's are all the generals, commanders and the like. The Ahamkara could make a return. They should have a nest somewhere in the mountains of Venus.