I would love to see config options to set mix block id/meta/size on a per-biome basis.

Biome config options for surface blocks are in the works now and will be partially implemented in 0.6.1

However, there are a lot of biomes, and... unfortunately, because of the way the surface system is setup, it will take a good while to get all biomes' surface blocks customisable. We'll probably start with all vanilla biomes, and then work our way through the mods.

Closing as duplicate of #322. This has been implemented for vanilla biomes and will roll out into mod biomes over the next couoke of releases.