I did try searching in case this had been suggested before, but I was unable to find any such previous suggestions. I may have overlooked some possible search terms, however, so I apologize if this is the case.
My suggestion is slightly involved, but I'll try to explain it as best I can. My suggestion is that dwarves have the ability to "restore" tiles that have been dug out into their former, natural state; probably their state from the year 1051, if this is possible. In order to make it clear how this idea might work, I've done some thinking into the details and come up with the following:
Step 1: The player would select an area for restoration, possibly by [d]esignation. In order to be restored, a tile must have previously been dug out, have no constructions present and have an adjacent floor tile in order for a dwarf to reach it.
Step 2: A dwarf would stand adjacent to a designated tile and begin restoring it to its "natural" state, whether that be a wall, ramp or whatever. If the designated tile used to be rock, the dwarf would need to bring a stone of the same type that the tile originally was, and use this to restore it. However, if the designated tile used to be soft, such as sand or soil, no material would be required in order to restore the tile.
As for restoring tiles that contained ores or gems, well, the dwarf would probably require a stone from the surrounding rock layer, rather than the ore or gem itself. Either that, or maybe areas which contained ores or gems could be unrestorable.
Step 3: When the job is finished, the tile would be exactly as it was when the world was first created, and the player would no longer be able to "see" anything that the dug-out tile uncovered. This means that the amount of uncovered subterrain would be decreased slightly, which means the player would have to do more digging somewhere in order to allow the limit on the number of created artifacts to continue increasing.
The reasons for my suggestion are as follows:
Reason 1: The main idea is that it would allow for the player to simplify the fortress (also presumably reducing the size of the save folder), and allow the game to run slightly faster as a result. Of course, the more you "restore" areas that are of no use to you, the more you'll take advantage of the opportunity to have the game run more efficiently. This would allow you to dig deep into the rock without fear of eventually bringing the game's speed to a crawl, as you can refill tunnels after you've mined out everything useful, and you can still dig wherever you like if you ever actually need more rock, or want rock of a specific type.
(I am working on the assumption that a region's save folder will actually contain the data that depicts the initial state of the natural landscape in 1051 (or on embark), and that as the landscape is then gradually modified from this date, during the construction of your fortresses, these modifications are saved accordingly. Of course, if it is actually not possible for DF to check what the initial state of a tile was by accessing the region folder, this whole idea is probably not possible right now. Can anyone provide information on this?)
Reason 2: Constructed walls and floors cannot be engraved on, as far as I am aware. Unless this is changed, having the ability to restore natural terrain could be desirable as it would once again open up the possibility to create new engravings in rock anywhere you like, regardless of any changes of heart you may have previously had about areas you dug out. (Or is it done deliberately that you can't engrave on walls in places you've dug out, to make you think twice before digging an area out?)
For example, suppose you want to engrave the walls of bedrooms to keep your dwarves as happy as possible. You may have dug out several dozen (or even two hundred) bedrooms only to realize there's a more desirable way of setting them out. However, if you dig out the walls that now form the bedrooms and construct replacement walls to reflect your new plan, you won't be able to have those bedroom engravings you wanted, unless you relocate the entire set of bedrooms. With the "restore" feature, you could simply get back the natural bedroom walls that you need.
Reason 3: If you make a mistake in mining, or simply change your mind about how you want your fortress to be laid out, you can effectively "undo" the areas you've dug out, rather than simply being forced to replace them with constructed walls.
The need to redesign bedroom layouts could be an example to support reason 3. Or, on a larger scale, you might one day decide that your fortress is not centralized in the optimum location for your current region. If you could "restore" to their natural state, things such as the central stairwell, main dining room, storage rooms, workshop rooms and so on, you could effectively move your fortress to a more useful location in the region, without having to leave your previous rooms unnecessarily open, taking up space in the region and lagging the game.
Thank you for reading through this long explanation (or even skimming it). I hope it was clear enough to be understandable. For all I know, it might not be possible to implement this idea at all, if the region folders don't have any way to recall the initial state of each tile on a map. If anyone can confirm or deny this, that would be great. Also, any thoughts or questions are welcome.
[ December 23, 2007: Message edited by: Tayrin ]