World Creation/ Multi-themed maps

[Karma]

I was thinking (and im not sure if its already in the config or not, I havn't found it if it exists) about a way to make multi themed maps. This idea could be very useful for various reasons.

When picking multiple themes it would be used in percentage so an example being:

Mountains-40%
River-10%
Desert-35%
Flat-15%

There could also be a distribution percentage for each theme which would distribute the themed layout throughout the map with the specified percentage. Each theme would get a default equal percentage of the map, so four themes get 25% of the map unless specified otherwise. If not using the default it could be laid out like this.

Mountains-30% distribution
Desert-20% distribution
flat-20% distribution
river-30%distribution