The title says it all.
In the embark screen, you are allowed to choose as many or as few world tiles as you want for your fortress. But this decision is
fixed for the entire life of your fortress. Why should it be? There have been a number of times where I wish I had added just one more row of map tiles during embark, so that I could, for example, dig further into the mountains in search of adamantine, or travel further into a forest for more wood.
( ^These are the "world tiles" I'm talking about)
Adding world tiles is one thing, but being able to subtract them would lead to tons of possibilities. Depending on how Toady programs the planned
moving fortress sections, entire fortresses could be mobile. You could then change a 3x3 map into a 3x4, move your fortress, and then change the map to a 3x3 (and so on) moving everything forward like an inchworm. This wouldn't even require a truly "mobile" fortress - all the dwarves could stay underground, with no permanent homes, moving slowly through the lands like Bugs Bunny. You could use this to build enormous things, like your very own Mines of Moria, a fortress which provides a shortcut underneath a mountain range.
There are a two problems I can think of with the ability to subtract world tiles:
1: If any dwarves are still standing on the part of the map you want to subtract, they would get left behind. This could be fixed by the game refusing to subtract anything until all dwarves have left the area, or by automatically shoving them to the new edge of the map.
2: Multi-tiled structures on world tile edges might pose a problem. If you have a 2x2 fortress, and you put a mason's workshop in the exact center, what would happen to the workshop when you switch to a 1x2 fortress? Would it be segmented? Would it be destroyed? Would it crash the game?
There are probably a few more problems,
(something tells me that rivers might not be too happy about this) and this whole proposal might be impossible depending on how Toady has already programmed things. But I thought I'd throw the idea out there.