Oryx probably has his own superiors in some fashion. Pantheon and all. Vex still have many more separate minds we need to take down. Cabal have an empire, necessitating an emperor. The fallen are pretty boned, idk what can be done with them now. Potential arc where we forge alliances with the fallen/rebuild the house of judgment with guardians and fallen? The Nine are certainly going to turn on us at some point. Xur's using all our vibrating coins to power some weapon of mass destruction or something. Fighting the nine would mean opening up the Jovians. We may have a raid on Uranus. Queen is love. Queen is life. We better not have to fight her. And finally because vanilla destiny didn't have a story we still don't know wtf the darkness is so at some point we'll have to fight it. Then there's the whole traveler is evil theory. Their well of bosses is far from dry.