One thing I don't fully understand is why people, when theorizing where the story of Warcraft will take us, it's assumed N'zoth will be his own xpac?
Both C'thun and Yogg-Saron were mid-xpac bosses. Y'shaarj was always only in the background or as an assistant to Garrosh. So we see all the previous examples they were never an entire focus of the story. Particularly Yogg-Saron.*
While he was the primary instigator and power behind Deathwing in Cata, N'zoth was a very minor role overall. "Old Gods" in general was the primary enemy, N'zoth almost never referenced.
While there's a lot of foreshadowing of N'zoth, and arguably has built up even more with Legion, I just don't see anything that even strongly hints at him getting his own xpac. And IMO especially true based on what we know from the Chronicles. The addition of Void Lords and possibility of creation of more Old Gods certainly fixes the issue of N'zoth being final Old God/void enemy. Now he can be killed without that storyline ending.
Do I think N'zoth will be killed in Legion? I think there's a very good chance but I'd still put Helya and Sylvanas/Genn plotline as most likely to be the mid xpac "side story".
*Yogg-Saron is the most interesting and relevant example because it was an xpac created entirely around Arthas. Not the Old Gods. He'd be the most direct example they could fit N'zoth in almost anywhere.