They wrapped up a lot of stuff they could have saved for expansions.
Helheim/the plain of death could have been an entire expansion, could have filled it with heart-wrenching cameos from departed NPCs we've loved and lost, featuring the rest of Yogg-Saron (the god of death makes sense to be somewhere in there), the intertwined souls of AU Ner'zhul and OG Ner'zhul fused as one and evil as ever, and of course Helya, but it would be cooler to get rid of her early on and make room for something much more sinister in the realm of death, probably Yoggy. Preferably a really awful cameo by Kael'thas doing a speech about setbacks just as his soul gets eaten by something.
Emerald Nightmare didn't need to be wrapped up so quickly, could have done Azeroth around the area of where the Well of Eternity used to be, mostly corrupted, the rest all fucked from the various Old Gods each etching out their own portion (as it was revealed in the chronicles all of them have reached into it to some extent, Yogg opening the original door). Didn't need to be huge sprawling forests and nothing but, and the red-black thorny aesthetic they showed us in Legion was pretty fantastic.
Argus could have EASILY made an interesting expansion. I'm not sure people aren't burned out on Draenei after WoD, but it would have been cool to have perhaps a chain of Legion worlds with Argus at the core for us to explore, all differing in theme and appearance. Instead of having "the army of light" which is already formed of many other worldly races and ready to go, have us as players rally it together, collecting stalwart survivors from the corners of Legion-owned territory. The priest order hall is already knee deep in the Nether and full of heroes, there may be other areas evading the demons as well. Also,
Lothraxion's people..?
I feel like I'm missing a couple. Goria in Draenor is one I'd like to see, but I feel I am alone in wanting to return to Draenor because of the foul odor lingering from WoD (If you want to see Goria in game, hit M and look at the map of Draenor, it's the continent in the lower left).
To answer the original question though, yeah I'd be surprised if it wasn't Naga or void related. Who knows.