The Minecraft world-creator program WorldPainter is cool, but I have an idea that I think should be added to it.
Biome Creator: A custom biome creator where you can specify ground material(s), underground material(s), tree type and density, generated structures, and more.
Biomes would be saved to WorldPainter permanently and you can send them to other people for them to use.
Ground Materials: You would be able to choose up to four ground materials to be in the biome and the percentage of the biome will be the specified materials.
Underground Materials: Same as ground materials, but for underground.
Vegetation: You would be able to check off which trees generate in your biome, and change the tree density. A little diagram would show what the density you chose will look like. It'll be a light green background with dark green dots to show where trees could generate and how close to each other based on you chosen density. Tree types include: Oak, Large Oak, Swamp Oak, Birch, Tall Birch, Spruce, 2x2 Spruce, Jungle, 2x2 Jungle, Jungle Bush, Acacia, and Dark Oak. You can also choose whether or not you want tall grass, flowers, sugar cane, etc. in your biome.
Generated structures: You can chose what generated structures should generate in your biome and how many Minecraft will try to generate per chunk. These structures range from ice spikes to strongholds.
Actual biome: You can choose what pre-existing biome will be generated where you put your custom biome. Defaults to Plains.
After you customize all this, you can name and save your biome! It will appear in the Biomes list when "Biomes" is checked off on the side bar. There will also be an option to send a custom biome to a friend via email.
I write books and play Minecraft, Terraria, FEZ, and whole bunch more games. I am also interested in animation and video & audio editing.