South Seas. But here's the thing, it could be both that AND Old God stuff.
Naga Invasions on every coast.
A faction of the Zandalari give themselves to the worship of the Old Gods/Void Lords in an attempt to revive their ancient empire.
Our "garrison" for this xpac is a ship that we command; it's crew are our followers. and we use it to travel between several zones.
3 raid tiers:
1) Nazjatar, end boss of first tier is Azshara (part of the questing is making pals with the Makrura in their city of Mak'ara and enlisting them in the war)
2)Zandalar, raised from the sea floor (where it sank during Cata) by a pact between King Rastakhan and the Old Gods (final boss of the tier is a Void-empowered Rastakhan)
3) N'Zoth's prison, Titan complex beneath the sea floor filled with all kinds of icky Old God crap; final boss of the tier and xpac is N'Zoth