It might be nice to see a return of the old aqueducts from 2D, for simple reasons of consolidating tasks.
Building floors across a gap, then walls on either side of the floors, can get more than a little tedious, to say nothing of the inordinate material cost(compare constructed floors vs bridges as it stands - and bridges can do more; the only thing they can't do is support construction). I think it'd be nice to have a "bridge" construction that is automatically walled, and costs extra stone accordingly. Like horizontal axles, its orientation could be changed; like bridges, it can be scaled in both dimensions if you want a wider one. Perhaps it'd be constrained to slightly smaller jobs - 8x8, say, so that the walls don't exceed the 10x10 dimensions seen in other construction. Doing it that way, rather than having the outermost two lines of the normal designation be made as walls, might prevent nonsense results - if you have, say, a 1x3 N-S aqueduct, and swap orientation, it doesn't need special-case programming to bump its width up to 3.
Though I mostly conceive this for aqueducts/magma ducts as the title says, it'd also be handy for bridging a chasm just so dwarves don't fall off accidentally, e.g. because a cave swallowman spooks them.
In general, bringing constructions more in line with bridges might be nice - have the option to make single structures, which, while not strong enough to be load-bearing, also don't take as much material.
This would probably require architecture to be reworked, though, given how slowly anything with variable cost gets designed now.