Given the known locations of C'Thun and Yogg-Saron, you can see by looking at the map of the Black Empire, that each Old God's influence and territory expands via tendrils stretching out from the center (just as their physical form does). With exception of course for Y'Shaarj, who was plucked from the surface of Azeroth creating the Maelstrom (the Southern remnants of his land later becoming Pandaria).
I've always just assumed that N'Zoth's name on the map was his location, but looking at the spread of his empire, that really wouldn't make any sense. He was the least powerful of the Old Gods, so it seems possible that the other more powerful beings encroached on his territory ... but N'Zoth is the weakest because he is the youngest, meaning that his territory must have expanded over or grown to meet the others in his vicinity.
Looking at the map in this way, it would seem that N'Zoth's more likely location is on the other side of the Eastern Kingdoms, either on a landmass or just off the coast. If you look for the "center" of N'Zoth's influence on the Black Empire map, it lies somewhere between Twilight Highlands, the Wetlands, and Arathi Highlands. Here is a description of the Twilight Highlands.
From Wowpedia:
In particular, the underlined bit sounds a lot like what is about to happen in BfA. We also know that Arathi Highlands will become a key staging ground for warfronts. I think all of the bloodshed on that soil is intended to feed something deep underground.
Going back to what I said about the name on the map being the presumed location of N'Zoth ... there is another map which pins his name in that location. However, the maps aside, all of the text about N'Zoth's location says it is a mystery. If it's a mystery, and no temple/prison has been discovered anywhere near it like with the others, I think that the maps give a false location. Maybe there is something there, some projection, but it can't be the true center because of all the reasons I gave above.
I think G'huun, or that other entity Shul-Naguruth is a projection and/or the accidental titan creation. Either they experimented with an outgrowth of N'Zoth and created a new beast, or it's another Old God we never heard of before. But N'Zoth must be off the Eastern coast.