It would be downright awesome if there was some way to export abandoned fortresses, and then add them to other worlds.
At first, I thought this would be an extremely difficult idea to integrate, since there'd have to be some kind of import system which matched up biomes, soil layers, river endpoints, etc, to the point where it would be downright impossible. Plus, I have no idea how games are actually saved, so the conversion process is equally beyond my comprehension.
But, actually, we don't need to make perfect matches in order to port an abandoned settlement into a square. Soil layers for an abandoned city are completely irrelevant -- as are any resources which were used while that city was still alive. The only important factor is the elevations of the outside tile edges.
Since the cities would be added during world generation, you could find near matches and then adjust the tiles surrounding them to link up rivers and relative elevations. This becomes even easier if it's possible to rotate the fortress layouts. After that, I think the only thing left to do would be to redefine the engravings to match the history of the new world being generated.
With a large enough pool of contributed cities, the game could even generate entire civilizations whose cities are designed by a particular player.
It seems like much of this functionality already exists in the Export Local Image command.
What do you think? What other steps would need to be added? Have i missed something important?