Hi Tereasa and welcome to the community! I love hearing other peoples biome concepts and mulling over the possibilities. An Underwater biome concept has been a recurring favourite among many users here on the forum, as with a Caves biome. And the idea of having blizzards in the Frozen North back in the Exiled Lands was quite popular too. I myself was luck that one of my concept biomes actually came to pass in this expansion, in the form of numerous Beaches.
You never know Tereasa, the Isle of Sptah is still only young, and most estimates put it at 75% of the size of the Exiled Lands map. This means that we may just see a few new biomes added to Siptah before its development eventually ceases. Fingers crossed hey.