currently there's 12 terrain types, and adding new ones should be easy. of course, world.dat files with the new terrains are never backwards-compatible after that, but who cares
in world.dat it's simply a matter of changing a byte to another value (should be easy as soon as Volutar includes that in his editor). Then we need a new texture for the geoscape, which should be easy, too. then somebody needs to make new tilesets for that kind of terrain...
I'm completely in favor of doing this, which is why I started
this thread some days ago. perhaps you should mention the "swamp" terrain idea there
(actually, the easiest new terrain to make should probably be "tundra/permafrost". there's a whole lot of it in Russia as well as Canada, and I think a lot of the "mountain" tileset could be re-used - basically everything except the mountains themselves.)