*Maybe not a bug but either way I have a suggestion to fix it*

When creating a custom world .json file, enabling structures without specifying each individual structure and it's parameters results in worlds generating with every structure that can spawn, in its respective biome, extremely frequently.

Steps to recreate:
1. Use the World Settings Generator: https://misode.github.io/worldgen/
*Found on the Official Minecraft Wiki
2. Click "Generator Settings"
*To change the generate file from "settings": "minecraft:[dimension]" to the customized settings.
3. Set "Generate Features" to true
4. Save and Import generator settings to a new world
5. Wait for an eternity for it to generate a mess of structures that you didn't want

Again, this may not be a bug(and is more of a suggestion) as the World Settings Generator site linked by the Minecraft Wiki warns that: "Not specifying a structure will cause it to spawn extremely frequently."

A suggested fix could be expanding the already existing "settings": "minecraft:[insert_dimension_here]" to allow us to mess around with each dimensions default settings. Similar to how the pre1.12 customized worlds worked.

Another suggested fix could quite literally be just bringing back the old customized worlds feature. That way we can more easily create customized worlds in minutes as opposed to spending hours(like I already have) messing around with a .json file.