We already have settings like ore density, toggleable forests, what animals we want to spawn, etc. But here are some suggestions I have for tweaking the world before its generated. I know some of these probably aren't possible and some will take so much time that it most likely won't be worth the effort, but I might as well get all of my ideas out there.
- Flatworld Height - Being able to change the height of the ground (minimum being a certain number of units away from the start of hell). This would allow people to build higher without changing the max build height. (ie. setting the ground height to -20)
- New World type: Islands. This would be cool for servers more than singleplayer. It would be awesome to be somewhat isolated.
- Ability to change the size and frequency (distance between 2 islands) of generated islands.
- Ability to change variation of island sizes
- Biome settings
- Ability to choose what biomes are generated in the world and what are not. (ie. no deserts, or all mountains)
- Ability to change how often a certain biome is generated.
- Animal settings
- Spawn frequency of each animal. (ie. more hostile animals, less passive = Harder gameplay)
- Being able to change the health of animals would also make the game harder. I wouldn't go with animal specific settings for this, but maybe animal group specific settings (Hostile, Retaliating-Passive, Passive.