Alright, this suggestion's probably not gonna go over well, because while it doesn't require a huge coding overall, it would change the actual gameplay a fair bit. But, here goes.
So, you know how dwarves mine out a ridiculous quantity of stone. I generally pre-mine out about a quarter z-level in the first few years, so I can flood it and start up a nicely protected, underground forest. I also made a thin, trap-lined corridor of stone inside a 40X40X10 hollow to repel invaders, still within the first few years. Both, on top of hollowing out a sizable fortress, threeish mining areas, an underground farm, automatic watering systems for the farm and forest, and expeditions down into the lower caverns and below. If memory serves, I had about three miners.
Now, I know digging around is the heart of the game, but that's a bit ridiculous! And I'm not the only one to complain that one of the larger problems in the game is just what to do with all the frankly unnecessary mountains of free stone you accumulate for no real reason. For that matter, I might not have the made strongest metal industries, but my pile of precious metals is still growing faster than I can use it. Of course, I can also think of why the dwarves dig so fast in the game, and that's because you start out with two-or-three moderately skilled miners and no weapons to speak of. If you can't build up a functional outpost within the first few minutes of the game, you're probably screwed, and you're definitely going defensive, not offensive.
On the flipside, there's also a pretty easy solution. Underground geology and ecology is a big part of this game, so why not have a lot more natural caves and cave systems connected to the surface? Just starting out, you could find an area with 90% of the work done for you, maybe just add a door and widen a bit in the middle. Maybe emphasize walled-off surface areas in the early game a bit more. You could get away with a slower mining rate, so that it isn't trivial to add another five z-levels to your massive pit just because your miners have nothing else to do, or accumulate metals faster than you can actually use them, let alone stone.