Probably the Cabal Emperor, some sort of Vex monster along the lines of a Time God. Cabal emperor/ shogun is a definite because they are just getting wrecked in our solar system. And then that was supposed to be it for Destiny One. Destiny 2 will probably release new enemies relating to some giant plot twist at the end of Destiny 1. Oryx will probably manifest himself through the Queen of the Reef or something. We may get to explore further out of the galaxy to other solar systems.