We took 6 expansion to seal Sargeras (but not kill him)... and in this moment no one on Azeroth is aware about N'zoth or even the Void Lord existens... So, even IF we kill N'zoth in the next expansion, doesn't mean we leave Azeroth again to face a Void Lord that no one knows what and where is.
Blizzard need some times to develope Void Lord figure as a great menace, bigger that Sargeras and the Burning Lergion. With that was easy, thank to Warcraft eritage but this, is something totally new...