What about this ? This requires minimum additional work as all graphics already exist, just redesign sets a bit:
* Desert (flat and mountains)
* Jungle (flat and mountains)
* Forest (flat and mountains)
With the exception of Jungle Mountain, all the other terrains are already done. There's already enough graphics for Jungle Mountain, so it's only a matter of designing the maps.
* Barren mountains (classic mountains)
The question is, how to use it and the other mountain versions on the Geoscape?
One option is to redesign the Geoscape to have textures for both mountain and non-mountain areas of the Desert/etc. biomes, which makes it more realistic. This option requires more work on the polygons and possibly adding additional textures, while making it clearer for the player which terrain will be encountered.
Another option is to have a single texture for each biome, and then randomize the terrain you actually get (basic, mountain version or Farm version, if available). This option would be less work on Geoscape redesign but would make the terrains more unpredictable.
Finally, it would also possible to create a Barren version of Mountain (without any elevations).
* Urban (same as terror but with ufo inside it)
In addition to Urban, there are several other terrains that can be used, like Industrial.
* Savannah/Steppe/Plains (classic cultivate, more natural blocks)
This one needs new graphics, and probably give it a more brownish tone rather than green.
* Grassland/Farm (classic cultivate, more human made blocks)
There's already an expansion to the original Farm terrain. One way I'd see it would be to have 3 Biomes with Farm versions: Forest, Savannah and Jungle. The original Farm could be the Savannah version, Native (included in the Terrain Pack) the Jungle one, and a new one based on Forest (using existing graphics).
* Tundra/Taiga (polar ground + trees from forest)
I was thinking of using something based on the Mountain ground, but it would need more color/vegetation. Tundra and Taiga are actually quite distinct since Tundra almost has no trees, but for the sake of simplification both could be used.
* Polar (flat and mountains)
Finally, Polar would really be limited to the poles, and Polar Mountain would only appear on the Antarctic. It would be very nice if the terrains could have versions according to the weather, but it is irrealistic to expect that this ever will be implemented.