I think one of the things Subnautica 2 is missing the most is cave biomes: I think the game would benefit both from giant open caverns that you can traverse with a submarine, as well as small narrow caves that you have to dive in.
This concept is for a large, open cavern biome, that I call the Spire Cavern. The cavern would be several hundred meters in length, but not in a typical cave shape, it would be a spiral-like corridor, that keeps going down deeper and deeper, potentially leading towards another, deeper, darker biome. Which is where the name comes from.
The cavern would have very little light in it, getting darker as you progress. Along the way there would be little fauna along the seafloor, with some bioluminescent sessile life, as well as mobile creatures lurking between them. Just like the sessile life, the mobile life in the area would be bioluminescent, to blend in with the seafloor. It could potentially include creatures such as the currently unused abbysal quadrant design, amongst larger predators.
Throughout the cavern, you would encounter plenty of large stalagmites and stalactites, with minerals forming on them into nodes, as well as sharp spike-like rock formations spreading out of the walls. The walls would be covered in a dark, scraped rock pattern, with a more sandy seafloor.
Overall even if nothing exactly like this gets added into the game, I’m hoping to see some other cave biomes in the future!