All underground generation, with maybe 10-20 blocks of space on top.

I would love to be able to generate a world, instead of 64 being average land height, 256 is the average land height and you could have caves all the way down to -64. Then I could finally live the dream of building a dwarven kingdom. Raising the average land height. Kind of like super flat, but the opposite direction.. instead of 2 blocks of dirt.. almost 99% underground.... I think this would be epic. Leaving just enough height to get trees, animals, villages...