So I've been playing the Dragon Age franchise on the side and **SPOILERS HERE** one the recurring themes is the Fade, a realm outside the inhabited physical one. It is home to the spirits and demons, and occasionally, from time to time rifts, or tears will occur. Allowing access to and from the spiritual plane.
Ny'Alotha falls into the same category as the Fade in my opinion. It's separated from the physical world of Azeroth via a thin veil. And portals or tears can allow corruption (the void) to seep into Azeroth.
From what I understand by reading notes from the 8.3 PTR, N'Zoth is defeated (wow, what a shocker, good writing Blizzard) but that doesn't mean his influence in WoW ends there. Patch notes reveal that currently N'Zoth is working to tear down the veil, and an expansion lead in could be that he succeeds. N'Zoth is noted as being the weakest of the Old Gods, so an expansion set on the resurrection of the surviving Old God souls and the rebirth of the Black Empire, given that now nothing separates us from the infinite realm of the Void seems like a "logical" (I use this word lightly) next step.
Now the matter of the Whispers of Il'Gynoth, the third death of whomever: Sylvanas, Azsahra, or Azeroth herself will help usher in the Black Empire. My bets are on Sylvanas, as she shows up but fails to stab N'Zoth with Xal'Atath and ends up paying for it with his last breath. This, I would suspect would draw us into the whole "Death subplot" as her Master communicates with us and leads us to Helheim or wherever.
So N'Zoth turns out just to be the bottom bitch that's lets the real Old God badasses immerge to kick our asses. Yay or nay?