Player built biomes.

This is only a small suggestion, but planting trees or plants one other biomes into other areas to create a new biome with those exacts plants would be nice. Even if It's small, there should be certain requirements met before that selected area starts “transforming” into that player's chosen biome (i.e., taking some spruce trees and planting them in your default oak tree biome to create a mini spruce biome).