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
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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...
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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
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(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.)