FOSS gamedev and creative worlds

I'm experimenting with biomes. Currently I have succeeded in replacing the stone/regolith/covering with various different types of nodes, but I can not find which parameters to modify in order to control the terrain features per biome.
I see that the only three biomes in minetest-game are "grassland", "desert", and "ocean". None of the parameters in the definitions of those biomes seem to control the shape of the terrain, apart from which nodes are used for "stone", "covering", and "filler", and the y-coordinate limits. Is the shape of the terrain simply defined inside mapgen, completely independent of the registered biomes? Or does mapgen structure the terrain differently in each biome, according to parameters outside of where the biomes are actually defined?
Do "biomes" even have anything to do with the structure and form of the terrain itself, apart from which nodes fill regions designated by mapgen as "stone" or "covering" or "filler" or "air"?

> Is the shape of the terrain simply defined inside mapgen, completely independent of the registered biomes?

Yes, in mgv5/6/7 'base terrain' generation comes first and places stone, air and water. Biome API later swaps out those nodes for biome nodes.

V8 (hmmmm's future mapgen) may have more interaction between biome and terrain, although that may be of the form 'certain biomes will only appear in certain terrain types' instead of biome influencing terrain. Apparently there might be multiple biome generators so you could choose your own combination of terrain generator and biome generator.

The problem with Minecraft is that biomes define terran type, and biomes are not defined by noise and have a very boring regular shape and size, so you end up with a regular patchwork of terrains, like a collage, there's no extra-large scale structure or 'big idea' like in *ahem* watershed or riverdev lua mapgens.

I think that the best option would be to make types of terrain generate in regions defined by noise, similar to the biome generator but on a larger scale. That way, an area of flat terrain could have grassland, desert, and forest biomes generated in it, but they would not extend into nearby mountain, ocean, volcano, or wasteland terrain types.
It would also be nice if terrain types could be defined and manifested in three dimensions - this would allow us to make certain forms of terrain generate in the sky or deep underground.