People forget that we have only seen a fraction of Nazjatar. We have only seen the palace, the immediate surroundings, and some of the ruins of Zin-Azshari.
There is an entire empire beneath the seas, not just a palace and some city ruins.
So it's not at all far-fetched to expand on Nazjatar, especially since Blizzard are keeping Azshara in store for future uses.