I have sampled numerous random terrain generators - and gotten some nice results, but I have in mind a tool that would be amazing. I have no idea how to create it, but if anyone had the know-how, it would be really nice to have a terrain generator that could:

1) Accept a real-world DEM as an input, and use that DEM to flavor the character of the generated terrain. Input a DEM from the Himalayas to get rough mountains, the Appalachians to get rounded ridges, Nevada for basin-and-range type topography, central Kentucky for sinkholes, etcetera etcetera.
2) Hey, a lot of real-world topography is defined by lineaments. Ridgelines in Nevada run pretty much north-south. I should be able to use a Nevada DEM for my terrain character, but change the lineament to east-west if I want.
3) Also accept a rough layout for features. You should be able to take a square, and draw ridgelines and rivercourses on it, and the generator would spit out a nicely detailed DEM that includes those features in the character of whatever real-world DEM you provided in step 1.
4) When one square has been generated to your liking, you should be able to match it to the edges of any squares created in the future. I've got a great terrain generated for this area, now I want to create a DEM for the area just to the west. I should be able to tell the software that and have it work.
5) I really don't care too much about rendering. There are plenty of tools for that. I just want to create realistic DEMs.

Rant over,
-Rob