More options for Customised worlds

A good way to manage the following list of ideas in the customisation menu would be to have a little plus square in a corner of a relevant button, that when pressed opens a mini-menu. Eg: For village-related customisations, a button on the “Villages: Yes/No” button.

Generated structures

• Village size slider In Superflat presets, you can choose the size of villages. This should also exist in Customised worlds.

• Village distance slider
  In Superflat presets, the distance between villages is also modifiable.

• Stronghold quantity slider
  Superflat presets also allow to select how many strongholds can be generated in a 1km radius from spawn. Perhaps Customised could also have something to determine what distance from spawn strongholds can generate in.

• Other
  There should also be frequency sliders for other structures, if possible (such as Ocean Monuments); however I don't know how hard that would be to code, whilst the three suggestions above are already part of Superflat.

It would also be interesting if there were options to change spawning conditions, such as biome, finding relatively flat ground (villages), or having an air pocket (dungeons).

Perhaps a menu to select possible villager trades would be interesting.

Another suggestion, which would probably be quite harder to incorporate, would be to allow to add custom structures and/or custom village houses with .schematic files (the format used in Mcedit).

Biomes

There is a biome slider in the menu that allows to choose either all the biomes or just one biome to be generated in your world. This can be pretty interesting (mushroom continents), but it would be better if there was an extra menu with checkboxes to allow to pick several biomes.

Another idea would be to also allow custom biomes, with sliders for which mobs spawn, which structures, which blocks, what the land looks like... but that would probably be harder to implement.

Ores

It should be possible to have a pop-up menu with an input box for the tries-per-chunk number, to allow smaller numbers (eg. 0.1). It would also be nice to have a per-biome option, to select in which biomes a certain ore can spawn (similarly to how Emerald only spawns in Extreme Hills).

Bonus Chest

It should be possible to choose what can be spawned in a bonus chest, or what will be spawned. This could either be an extra menu with the list of things that naturally spawn in bonus chests, and boxes with maximum and minimum numbers (or sliders, like with ores). Something similar with dungeon chests would be interesting.

Other suggestions