This was ment with "multicreation". I would implement something like this with another internal skin for the terrain. Everywhere the skin has a color, a random mix of trees grass etc. is placed. Of course this could be extended with a algorithm that takes care of where the beginning of the forest is, and placed smaller trees there.
Additionally you could add different colors, red means mix1, green means mix2, etc.
The skin could be invisible and only used for internal purpose, but of course it also could be visible for the user. I would call it some kind of vegetation map then.

I know that these features are not the most important features at the moment, I just want to share some ideas.