I can only find this: Multiple biomes (rock, earth, grass, infected, temple, and much more) in the description so I couldn't tell which biomes you already have and which you haven't. I'm not even sure what infected even mean or look like.
So I'm just going to throw ideas here :
- Bones. Not just skulls, but bones everywhere from floor, wall to ceiling.
- Crystal. Small and large crystals everywhere.
- Coral. Floor, wall and ceiling are made of corals.
- Wood or Tree. Floor, wall and ceiling are made of tree branches.
- Bee Nest. The inside of bee's nest.
- Organic. The inside of giant beast's body.