I'm thinking Cabal, since it was foreshadowed in TTK, but there could be a new enemy at play, and the Vex could have a part.
I don't really think it's the Hive, since their leader is dead and they were already the main antagonists of Destiny's DLC.
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Oryx still has 2 sisters. But he was the most powerful, so it'd be a little underwhelming.
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It's mentioned that they actually could have become stronger than him, due to sword logic.
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True, but I was under the impression that he got a colossal power boost when he killed Akka.