A Cold biome with ice, snow and maybe frozen lakes.
A Water based biome with crabs, lobsters, shrimp and other water based arthropods.

Even if I like the idea of a cold biome, I'm not sure if it will fit the tropical warm humid ambience of the Erebus. But I definitely agree for a sea one.

The Erebus to me is like a life-filled cavern deep within the crust of the Minecraft world, where temperatures are much hotter and lava lakes appear frequently. A cold biome would be very out of place here, but I believe something much more special and unique would fit: Crystalline biome!

Such a biome should probably be somewhat rare and challenging, possibly with the risk of lava or powerful enemies around every corner, but it could be the most beautiful and rewarding area to explore.