I'm gonna use the new Y limits to make a huge Ravine with caverns sprawling everywhere. Multiple layers, the bottom being crystal caverns, above being caves, mineshafts, maybe even a dungeon or two? Then, above that the actual ravine, maybe a cliffside village? The top being a forest biome with a nice park to walk around. Or maybe that'll change? I'm not sure what biome I want to do on top, but definitely nature. And with the huge Y increase I wanna use all the new lighting blocks to make stars.
