I bet it would be hard to make real but it would bring more gameplay elements and It'd make the map more sexy.
The terrain is already made up from "blocks", sorta. You could make layers above it to create a little hill, one more makes a bigger hill, and if the middle layer is hollow you get a cave! (The upper layers should become transparent in a radius based on light)
Also it would be nice if the player could research tools for terraforming. Creating hills, dig caves. (imagine that you dig one layer below then you connect that digged part with a lake to do a "reversed" landfill.
Making caves for your train would be awesome too.
And also, to compansate for the terrain based walls, bigger, evolved aliens, or even entirely new types (jumper, carver) could dig through walls/terrain or just jump 1 or 2 layer of terrain (based on evolution)
Oh, and as always: Sorry for my english!
