Edit: I also just rewatched the features trailer, and it got me thinking- is the Voidstorm actually another piece of K'aresh? The first thing we see of it in the trailer is another Ethereal Manaforge surrounded by Ethereals. A Manaforge without the NPCs is also shown in this image:
The less-twisted bits of land we see there also looks similar to K'aresh. And, since all we've been told about it is that it's a mysterious distant world shaped by the Void, maybe it could be another fragment that was fully swallowed by the Void?
With that said, it could also still be another world the Void consumed, and the Ethereals and Manaforges could be new arrivals after Xal'atath captured Salhadaar. If that's the case, maybe it's Xal'atath's homeworld?