This is a fairly small and simple suggestion (especially considering it's me). I'm surprised I didn't see something exactly matching this in the searches I did, although I did certainly see plenty of suggestions for explosives use.
The basic gist of this is that we get a new designation type ("sap wall"?) that lets a dwarf carefully undermine a wall, and prop up the holes with small collapsable supports (possibly taking a wood or bar or stone, like with a full support). These supports are then capable of being attached to a lever. The lever, once pulled, like with a full support, will collapse that wall, while keeping your dwarves from a safe distance.
This idea comes from my general reaction to the almost suicidal way you have to engrave fortifications in the side of magma pipes to gain access to them from lower levels, and the fairly dangerous way you might otherwise "break the dam" by channeling right next to the places where you are about to channel water. Collapsing walls like this wouldn't require any explosives, and give players a safe, if time-consuming way to bust walls next to dangerous areas.
While I'm talking designations, though, even though I know it has been suggested several times before since .31 came out, I would like to reiterate that I would really like to see a "remove construction from above" command. Even if this is only usable with stairwells, this would be a useful remedy to the problem of having ramps down in all your pits. You could just remove the ramps (from the lower floor), build a stairwell up halfway through that, and destroy the bottom of the stairwell from above, so that the pit has a smooth bottom without the absurd extra steps of collapsing a floor onto it, or digging an escape route you then have to wall back in. After all, if we can build stairs downward (as a replacement for a rope ladder), why can't we deconstruct the same object downward, as well?