Minecraft includes a lot of abstraction for chunk generation that is not included in the CraftBukkit/Spigot API. Currently to achieve similar world generation one must:

a) recreate the entire abstraction system using block populators

OR

b) use NMS

A lot of modded platforms (Forge, Fabric) allow the usage of the in-built minecraft carvers, features, surface builders and structures, giving access to use these/create our own of these, in most likely addition to creating dimensions (possibly) would improve the world generation API in Spigot, that hasn't really been updated since Bukkit's creation. I believe it is time to update this part of Spigot's code. 

In general this issue is asking for an abstracted class for Carvers, features, structures and surface builders that can be injected/added to a WorldGenerator, as well as an ability whether it be an enum or a statically initialized class containing the vanilla carvers, features, ... , to add to a world.