I think there should be a few new world type options, here are a few ideas
Full Biomes - Entire world is one biome which you choose
Spawn Biomes - You spawn in a biome of your choice
Skylands - I know this would of been already suggested but I think it is still worth mentioning in this thread
Volcanoes - The world generates volcanoes naturally, also there will be more exposed stone and a lot more lava pits
I don't think the gravel would work in the game mechanics, maybe fire charges instead
I think some of the volcanoes could be stone and some could be dirt and covered in trees
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Creeper Hugging, Its fun but it might be the last fun thing you do
Oh, another idea. A world where it is all large caves, the surface has nothing worthwhile but underground there could be caves with tree's growing from the light that shines through the holes in the roof. It would be awesome
Full Biomes - Entire world is one biome which you choose
Spawn Biomes - You spawn in a biome of your choice
Skylands - I know this would of been already suggested but I think it is still worth mentioning in this thread
Volcanoes - The world generates volcanoes naturally, also there will be more exposed stone and a lot more lava pits
If you have any more ideas please comment
I think some of the volcanoes could be stone and some could be dirt and covered in trees