This is more of a long-term idea. Possibly even multi-generational.
I propose going back to the Dragslay ideal at some point, and re-implementing the game as a continuous 3D world without tiles, z-levels, or other discontinuities. This sort of has the city-builder game Foundation as inspiration.
I'm trying to imagine the changes to mining and construction. While designating, corridor width and height would be fixed, but elevation angle could and should be easily controllable, with quick defaults available: down, angled-down, horizontal, and angled-up. I assume corridors would always be rectangular solids. For rooms (i.e., non-corridors), I imagine they would always be prismatic solids of uniform height with floor and ceiling horizontal, but arbitrarily-drawn perimeter. Intersecting rooms and corridors could create more elaborate terraces and so forth, involving the same sort of labor workflow as now, mining them from top to bottom and/or constructing from bottom to top.
It would be nice to create quarried stone from carving out these designations in some believable way rather than having something pop into existence. Maybe each pick strike could be at or near the edge of the current mining direction, and each strike around the perimeter of this direction has to fail to crack the stone in the middle. As soon as it cracks, we go into "who cares" mode and whack the nearest stone bulge, which crumbles and disappears. If it doesn't crack, the new raw stone cube (because what other shape would it have?) thuds to the ground and the miner walks around behind it and continues. This is a lot of attention to give to one game mechanic, but it's such a core part of the dwarven mythos/pathos that I think it's warranted.
Internally all walls and surfaces would use minimal geometry, but natural cave, rough-hewn (which should be different from natural cave), smoothed, engraved, rough block, block, engraved block, etc. status would be conveyed with bump-mapping. Since there are no tiles, smoothing and engraving would be on a per-surface basis.
Constructions would be composed of either some very simple parametric shapes or pre-figured building parts included with the game as a resource, placed to intersect. The final shape of a construction would be the union of its placed shapes during designation, though each shape could be constructed separately. Each shape would have a (possibly optional) interior, and the final construction interior would be the union of its shape interiors. And it would be possible to designate mining through constructions just like through living stone, but without the possibility of quarrying.
Well, that's enough for now. Fun to think about, anyway.