Is there any way to create a mod that will create a Random Map of a specified size that is guaranteed to have certain terrain features.

Lets say I want to make sure every game i start is going to have a capitol and kings castle (obviously would need a custom map) as well as a portal to the one of the elemental plains. the wrinkle is I also want the map to be mostly random other than those few terrain types. I have been playing around in the map editor and there are the 2 ? types of terrain, however, if i use those I know the shape of the map still.

I cannot figure out how to use either the map editor or pre game events to create the desired terrain types at launch.

Thanks for reading my ramble