Imagine your underground adventure starting in the biome of your choosing! It is scaled for difficulty, and the Dirt biome replaces it wherever it occurs naturally in the game, also scaled for difficulty.
I'd start in the Sunken Sea. I think it'd give the game a really interesting new vibe as a starting out point, and you're essentially trapped there until you figure out how to craft a boat. I also think replacing the Sunken Sea with the Dirt Biome as one of the biomes you stumble upon when the wall comes down would be super interesting. What kind of creatures would you encounter if they had to be as strong as the Bubble Crabs or the Caveling Scholars? Imagine a Glurch as powerful as Omoroth!
