Yes, I agree that it would take a whole lot of work to make a third faction, so it probably won't happen.
After all, in order for it to be balanced (and we all know how much Blizzard loves balance) there would need to be PVP for all the different brackets - though you might be able to just hijack the existing ones.
Also, inventing new cities and what-not - I hoped that would happen when they made Outland, but alas.
During patches between Before the Storm and now, they have added new quest nodes, new flight points, graveyards, cities, etc, so they could probably do it if they decided it was worth it.
After all, I remember when Forest Song in Ashenvale was nothing more than a dryad and a moonwell.
Ah, well, maybe I'll never get to play a naga - at least I can still Mind Control them and pretend . . .