Superflat worlds have been around since Minecraft 1.1, and now they are about to get a tune up. Mojang developer Nathan Adams posted this screenshot on his twitter earlier today with some of the options that will be available. This screen currently allows for customization on what blocks are on what layers. His words in the tweet were, "No clue how I'm going to implement the "choose a block" part yet." I'm just hoping you can turn off slimes spawning.Source: @Dinnerbone