Either N'Zoth or a Void Dragon (ala Vexiona) can serve as the box villain, not too differently from Deathwing in Cataclysm:
Which does make for a badass cover.
The story can revolve around a reborn N'Zoth (either reborn via the Blade of the Black Empire or we straight up never killed him in the alternate dimension associated to visions called "Ny'alotha"), who is threatening the Dragon Isles, trying to create an army of Void-twisted dragons, or looking for the next Plot McGuffin button, or something like that. It really doesn't matter.
I would also like to remind the gentlemen and gentlewomen here that the Dragon Isles were mentioned in BfA by Wrathion's agent and that Wrathion was "looking for it", and it was never revealed "what" he was "looking for" in the "Dragon Isles", or why he was so interested in those isles in the first place:
If Wrathion was looking for the Dragon Isles, why not N'Zoth? And could they both be looking for the same thing, what is that thing, and for what purpose?
Easiest expansion to set up, the build-up is there.
And I will gladly welcome the inevitable Ren'dorei screentime that comes with more Void-related storylines
