I'm thinking I want to go about 40/60 on above-ground and below-ground area types. A bit less than the first third of the game will be an overland trek through a fairly linear series of woodland/badland maps, then there'll be a fairly long stint in some underground caverns, another short stretch overland (through volcanic badlands), then an even longer stint in caverns descending downward toward the buried ancient city for the ultimate battle with a Maugrim Lord. This final stage will likely comprise 3 regions: caverns, mines, ruined city. Perhaps a subterranean volcanic region somewhere in there as well, below the ruined city.
I'm thinking I probably will not implement persistence of levels. Backtracking through different areas will likely not return you to the same maps you visited before. There is a precedent for this in the lore of Golem, you see. Back when it looked like humans might actually win the war, the One Golem unleashed a terrible form of chaos (the Maugrim) whose power was sufficient to twist the laws of reality. Called the Mazing, this cataclysm imposed an extreme disorder on reality, causing things to twist and warp beyond all predictability. With the chaining of the Maugrim, the Mazing has been reduced somewhat, but there are still places where it is in strength, and Ture lies at the heart of one of these Mazed regions.
Not to mention, it's so much more fun to be able to go back and explore new levels every time, rather than merely backtracking through the slaughtered remains of dead enemies and thoroughly plundered (thus, boring) levels.
I've started collecting existing terrain sets and sorting through my archives of digital photos for additional sources. I'll probably head out into the desert this weekend for some more rock, gravel, sand and dirt photos. Can never have too many. [grin] Over the next several weeks/months I will be carrying that damned camera with me everywhere I go, taking photos of whatever random objects and textures I come across. Yay for digital cams. (I should probably invest in a sturdy carrying case for it, though.)