So at this point it is pretty much confirmed that the void will be involved with the next expansion. I mean every hint points to it and absolutely zero things point to any other direction. Question is, how will the new void presence manifest itself?
1. Azshara
The most likely one. The naga were absent for MoP and WoD but Azshara is pretty active now and we have naga on both sides of the conflict who by the way do not hide that any cooperation with us or the Legion is temporary and they are loyal to their queen only. QA was the most powerful mage on Azeroth, who made Mannoroth shit his pants with only a look. And that was before her transformation into a naga. She is quite obviously the strongest tool N'Zoth has. Confronting Azshara and the final Old God is long overdue.
2. Emerald Dream
Yes, I know the nightmare was defeated several times already. But we still did not get a proper ED expansion. I mean one layer should be the size of Azeroth, with several layers available. This could even tie in with the naga expansion. I mean they were former night elves. Maybe the Naga find a way to enter the dream directly and mess it up.
3. Northrend 2.0
Since a lot of people want it, why the hell not? While Yoggie bear is dead, N'Zoth could perhaps send a few disciples to cause some trouble. What if the Lich King himself becomes void corrupted? Or one of the Dragon sanctums? Or Both? Void King sounds ike a pretty good title.
4. Voidgion
Sargeras is absent from the PTR. Maybe to avoid spoilers, but come on. We have void Alleria and the return of all the titans. It wouldn't be much of a spoiler now. Unless the reason he is not in cause he will be the final boss of the next expansion. What if Sargeras himself became corrupted by the void? Supposedly his plan is to turn the Azeroth titan fel corrupted, which is a very Void Lord thing to do. Of course a purple legion instead of a green one is not the most imaginative idea, but hey. Blizzard.
5. Argus
We deal with the legion but all that void corruption we see on the PTR spreads and the whole world becomes a void infested nightmare. After we do the 7.3 storyline we loose access to the small green Argus and get access to the large purple Argus of the expansion.
6. A new planet filled with void stuff
What the title said.
7. Darkmoon Faire
Seriously. That whole place reeks of evil. Silas Darkmoon is up to something nasty. After we kill him the Faire is taken over by his less void infested half brother or half sister to make sure we don't loose access to a seasonal event.