Making terrain can be a tedious job, I think a tool, especially for terrain manipulation, would be a great addition.
-raise and lower terrain on a plane
-sculpt land masses
-texture painting, paint on the side of a landmass with a texture that looks like rocks to make it look like a cliff. Or gravel, Stone texture for a road or path.
I think it would be a valuable addition to the game and I'd welcome it openly