So I saw that the caves and cliffs updated was announced at Minecraft Live this year... and I though I would would suggest a cave biome! So the Sub zero depths would be a very very cold cave that generates under cold biomes such as mountains, and the snowy tundra! These caves can be very difficult to travel through because there would be lots of ice hanging from the ceiling that would fall onto you when you walk by! At the bottom of these caves is lots of obsidian because the lava and ice collides! Sea lanterns and crystals could generate too! The entrance to these caves is extremely narrow but when you reach the bottom of the cave, you find a huge and wide open area! This would add a really realistic experience to the game!