Advanced Surface Rules & Depth for biomes to generate

For now you can only edit a biome's grass block, dirt block and underwater block. That means, we cannot get proper layered surface generation like sandstones in the desert, and have to use features to imitate that. 
However, proper layered generation is possible even in hand-written Datapacks for custom dimensions, through Surface Rules. Therefore, I suggest to include a Blockly-based Surface Rules editor for biomes, while the simple grass-dirt-underwater type is still available.
Also, for now, we can only generate underground biomes in the overworld. By letting the modders to edit the depth for biomes' generation, this can be made possible in custom dimensions.