I've been entertaining the idea of caves-only survival challenge (ie. no forest world, just caves), the biggest reason probably being "the caves florid postern finally getting some use". 
Upon closer look, I realised how many items I consider essential in a full world would become unavailable:
- All tier 2 and tier 3 warm clothing items require something only available in the forest (beefalo wool, koalefant trunk, thick fur), leaving me with rabbit earmuffs and a thermal stone to survive the winter.
- Is there another way of getting rabbits in caves besides waiting for them to drop during an earthquake? This is going to put a serious damper on being able to make magic items.
- Considering the above, any early revivals will have to be at the portal, or with a telltale heart, and both have a max health penalty. Is there any way restore max health? No bees -> no stingers -> no booster shots.