Oryx's sisters are still out there somewhere, and may be more powerful than he was (but also may have different motivations). There's also the matter of the Hive Worm Gods, and their possible relation to the supposedly extinct Ahamkara. I doubt we've seen the last of the Hive, but it may be a while before the focus swings back to them.

All that said, might we see a new enemy? How relevant can the Hive and Fallen actually be in D2? All Cabal and Vex makes D2 a dull boy, no? Yes? No?

(Preface...I don't know much about the lore, these are mostly observations)

I'm curious about possible alliances. The Fallen are basically pirates and don't really have a "home". Variks hangs out on the reef and Mara Sov had Fallen servants. All those reef people are kind of strange anyway. Could there be a Fallen/Awoken alliance that in turn compliments VanGuardians?

There's just so much I don't know about that I'm really curious about.

The Cabal are just marauders that absorb worlds for resources. Are they interested in Earth's resources? The Traveller? Both? What's their main motivation for attacking Earth?

I know nothing of the Vex and their motivations.

It seems to behoove the Fallen and Humans to form an alliance against the Cabal and/or Vex in an effort to protect the Traveller. We've wiped out Kells and Priests and Archons blablabla, they are weak. Now we are getting blindsided and we are weak.

[cue Ragashingo]

Right. Oryx's court and daughters and son (Crota) are all likely dead, though there's a small chance the Daughters and Court might come back (like Omnigul did) as we didn't really go through the long involved process of hunting down their Throne Worlds like we did for Crota and Oryx...

Oryx's sisters, Xivu Arath, who liked warfare and brute strength above anything else, and Savathûn, who was clever and deceitful above all else, both left Oryx after they all three together destroyed the Harmony and their Gift Mast. Both are still out there somewhere, though.

Savathûn in particular seemed to have major second thoughts about Oryx's strategy of winning by being the most powerful, most destructive thing there is. Her last big action was to perform experiments on the Harmony's wish granting dragons, which are likely closely related to the Hive's own worm gods. That she turned away from the philosophy that both Oryx and the worm gods talked up might mean something going forward.

Xivu Arath doesn't strike me as being nearly so bright. I think she went separate ways with Oryx more because she was tired of waging war for him only to have him be the one to strike the killing blow against whatever race they were fighting at the time.

My Rampant Speculation would be that we won't see Xivu Arath or Savathûn for a while. But, although Oryx and Crota are probably as dead as they can be, Oryx's Dreadnought, and not even Oryx himself was/is perhaps the real threat. Remember, its hull is made out of the "dead, but still very much alive" worm god Akka. And it was certainly carrying tons of baby worm gods and I seem to recall that you could scan one of those tubes and were told it contained a Hive worm... if that's right, the ship is carrying a bunch of superpowerful worm gods and is itself a "only mostly dead, which is still very much alive" giant worm god!

Oh, and heck, Oryx himself was host to a worm god that had been feasting on destruction for hundreds of thousands of years... and we just left him and it floating out there in space!

See Also: Bite-sized Backstory 25: The Harmony and the Ahamkara