First off, I'd like to state the same thing I did with my earlier posting of multiple ideas here(Flaming dwarves) that I did not check every idea here. If even ONE is original, its worth the extra topic.
Explosives
Yes, I know that there is the dwarven "Booze Bomb" but that requires magma(To the best of my knowledge) for it to work. So, I'm thinking why not add an easier to use explosive? It could be a building that is set off by a lever or a pressure plate or something, made at an alchemist shop.
Constructions
Something that really bugs me about floors and walls as they are now is: You simply CANNOT build a wall on a floor. I don't see how this makes any sense at all, would a floor REALLY interfere with building a wall? No, probably not. This gets really irritating when building aboveground structures, like a house.
Another thing that makes aboveground forts difficult: You can't engrave on constructions. This means that outside houses need to either be bigger, made out of rarer material, or have more pricey items in them for them to be more valuable rather than just being able to engrave the walls.
Workshops
This isn't really a big issue now that dwarves can move diagonally, but it would still make workshop areas look better. Why not be able to rotate workshops? Before they were able to move diagonally this really fucked me up, I didn't pay attention to where was blocked and got them trapped before. While this can still happen if a building is placed incorrectly, its not as likely. But it would look nicer if you could make it so the counter in the jeweler's workshop was up against the wall or something like that.
Wagons
A problem that I've had before when making a fort is that I almost always make sure to make the depot wagon accessible ASAP. What bugs me is, you can't ever be sure just where the merchants come from before they show up the first time. Why not point this out at some point BEFORE they shot up?
Machinery
I've been using the screw pump recently, I figured out how to automate things not that long ago. One thing that I'd like to expand upon is being able to bring water up more than one Z-level. This would make moving water much simpler, though it should require more energy than a screw pump.