The following can be placed into events:

Crystal Fungi - Can be moved to the SHROOM Event Type
This event will check for cavernous and non-cavernous shrooms, and decorate as such. Cavernous can only be placed on Stone, while non-cavernous can only be placed on Grass. Probably per Material, not per Block.

Crystal Blooms - Can be moved to the FLOWERS Event Type
Just decorate Flowers like Vanilla would, I guess...

Sugar Cane - Can be overridden with the REEDS Event Type
Just get rid of them.

Actually, scratch all of the above (even though I won't). Instead I'll go ahead and make a new Biome Decorator that does not piggyback from Vanilla.