Because the area to the north of the Coral Gardens region is too far gone to suggest anything for due to the Scopius region being built there, this is for expansion south and west. South of the south raceway, we could have a new shallows biome- One with roughly the same flora and fauna, however it would be deeper, have design similar to that of the original steam screenshots, specifically the ones that feature the wakemaker and the scanner; and feature actual coral crabs in it. The area could slowly decline into a dark colored rock zone before (in the west) blending into a deeper expansion of the tufa towers biome.
Expanding the blighted coral zone west, we could see a dead or dying shallows biome similar to the eastern edge of the South Raceway (without the coral crabs) and have young necrolei jellies growing in the wasteland- The biome could wrap north and blend into the true Necrolei Hills. No new creatures or biomes needed. Just expansions and new instances of old biomes. This would make the game so much better than just keeping the plateau drop off suddenly a couple hundred meters everywhere and having new stuff at the bottom.