Could you please provide the necessary hook points?

That's the issue...I can't figure out where they are or else I'd do it myself :P

I've looked into the problem domain, with help from Ansraer.

Short version: WAAAAAAAAAAAAAAAAAAAAAAAAAAA

Long version:

• The BiomeGroupToBiomeLayer class is the one which determines the initial biome sampling. However, the way it's designed means that you can't safely just add custom biomes to the list - the arrays are static!
• We'd most likely need to add hooks after every layer in the game, allowing contextual modification, filtering and registration of your own layers. This is actually pretty hard.

At this point, I think the sanest solution... I don't know.

In general, I feel that the vanilla world generation mode should be left alone, with an extended world generation mode being added by either library mods or started (but extendable into custom ones) by API itself.

@Prospector Is this still a going concern?

Wait... this was a thing? :P