I've seen all kinds of crazy theories about N'Zoth's location: from the Tomb of Sargeras and the Maelstrom to... Deepholm. Because "the circle of stars" can't mean anything else, right? But the thing is that one of the maps from Chronicle vol.1 (2016) clearly shows the location of the prison prior to the Sundering. And if it didn't get a Kul Tiras treatment with some tectonic plates shenanigans, it has to remain in that very place - which is somewhere around modern Vashj'ir. A video comparison of the maps for those interested in the topic.