I love using Worldgen's importing feature, along with graphics programs, .DEM data, and erosion simulators to produce worlds for DF. There are a few features I have found myself wanting very badly, however. None of them are too complicated code-wise, although some might require a bit of "trickery" and "Dwarfiness" to accomplish with the current code base, but nothing too drastic I would think.
1) Easy things first. I want to be able to import good/evil maps. You know it's simple to do if the framework is already in place for everything else, and exports on a detail map in the same format as everything that you can export.
2) A little more interesting. Oftentimes I create a feature on a map that is so ominous, so perfect a place to put a volcano, a cave, or maybe both. Lairs and Civ starting sites, too, sometimes beg for that perfect location. If not the ability to specify populations, I would at least like to be able to set "spawning points" for various things:
a) Civs - with or without being able to specify WHICH civ
b) Megabeast caves - worldgen misses many truly fearsome geographic features that would make good atmospheric surroundings for something like a bronze collosus or dragon's horde.
c) Volcanos - this is only semi-possible to control by setting 100 vulcanism and number of volcanos, but a way to say "here there be magma" and be done with it would be very nice.
d) Caves to the Underground - It would be nice to intentionally pierce the earth in certain spots, as the engine allows, with deep caves into the cavern levels.
3) Wishful thinking. Underground maps. This probably isn't modelled yet as something one could control, but I think gradually varying the openness and cavern density in the same way that a parameter like temperature or drainage is, would be a great addition to the way worlds are generated and displayed. Coupled with the vulcanism maps already in the game, it would be possible to export (and if you follow my reasoning on the above suggestions, import as well) maps of the varying resulting open spaces. The idea behind this meshes very well with the 3d minerals concept I've heard discussed on DFTalk, and it looks like this area of code is already being looked over.