Thats not really how the game works, the biomes are ingrained deeply into the simulation eg, swamps are based on a salinity maps, water maps , temperature, climate etc and a ton of other maps on the world map.It doesnt just randomly place biomes they come about based on climate and stuff like real life. If you mean defining something akin to a"good/evil" biome the plan is to base them off of spheres. I doubt we will be able to create our own, we might be able to edit raw files for them like with evil biomes and stuff though.
Ahh, big shame then, I was thinking something akin to an evil/good/savage biome setup, just with being able to define only certain types of grass/rock/etc. to show up in said biome, instead of amid the generic types of material.
It's not like biome definition is something that
can't be moved to the raws (for example, a mod could shoehorn subtropical ranges between the tropical and temperate ones), but it would require modifying a lot of vanilla raws to have any impact, and doesn't seem worth the effort for the foreseeable future.
If plants and creatures eventually transition to preferred environment parameters rather than named biomes, then the biome descriptions themselves become flavor text and moving them to the raws makes perfect sense. Then if you really do want to build your little house on a Prairie, you can.