You could also shoot a cage at someone, but I'm guessing it'd be pretty inaccurate.
Reminds me of
Project Pigeon... Not actually a suggestion, but, thought it worth mentioning.
Or load it with an *empty* cage and turn it into a cage trap ...
This would address something I have a problem with, with cage traps. Because they are essentially non-blocking, I sort of envisage cage traps as being cages (necessarily originally manufacturered so that they can be 'folded out' upon the floor) snapping up around the victim. In bush and shrubbery areas, I could see that being disguised, but on bare rock (or, worse, smoothed/decoratively engraved stone) that leaves a big wooden (or metal/etc) cage on the floor. I don't see it being a suspended trap that falls, in the open air, although I suppose one built inside could be suspended fro the ceiling (at least until that's deconstructed) so mix-and-match. Maybe traps should need natural surface on the tile floor, or a roof above. (And if either gets changed, trap deconstructs.) However, that's just a thought that ties in with this suggestion.
Hand Crank
As a loading/priming mechanism for traps, or are you adding that for mechanisms in general? (Although the only non-trap mechanical application I can currently think of using a hand-crank for is
remotely powering a pump, where one hasn't had the foresight or opportunity (e.g. dangerous external environment, and a shortage of water for a Perpetual Motion system) to link wind or water-powered rotary power-grabbers down to a remote pump in a hazardous area that you don't want your dwarfs anywhere near.)
Pressure Plate
* Same as the current one, but also be able to trigger based on the *weight* of items on its tile.
I've not messed with modern (2010) pressure plates, but IIRC there's already creature weight, as well as the traditional water/magma depth. Although being responsive to things like rocks would be interesting (which don't currently do any damage, outside of a traditional specifically constructed stone-fall trap?) and consider a variant on the 'cat randomiser' based upon where a catapulted stone lands. (Actually, consider an automatic training device, where you could have the dwarf locked into the catapult area until they get three straight hits at maximum range, with appropriate combining.)
Timer
I think I've previously gone for the idea of in-line-delays being a two (or three) mechanism black box, constructed at the mechanic's shop before emplacement. Although thinking about it in hindesight, the quantum interactivity of remote items perhaps make it more relevent to use a delaying mechanism(-combo) instead of a normal mechanism as part of a standard two-item selection for mechanical linkage. Unless you want them to be dwarf-adjustable. (Then as well as lever control rooms, you might have an adjacent (or anywhere!) timer control room set up.) But that's just waffling thoughts.
Suddenly you have a semi-automatic catapult or something. Pity no one knows how to aim these things.
Another mechanical linkage idea, remotely aimed catapults. Could be attached to an axle (actually, probably needs that to power the actuator) but otherwise lever (or pressure-plate/etc) controlled. Either an input turns it position 1 or position 2, as set on construction (a second input allows positions 3 or 4?) or a clockwise-setting input makes it turn that way one step (another input could be set for [anti-/counter-]clockwise, or just pull several times. But in the former case, the semi-automatic catapult can be made to trigger to defend whichever bit of corridor a pressure-plate has been triggered in.
Enterprising folks might even be able to use it to haul stone up a few levels, somewhat like catapults have been known to do (except that they can't send it to a higher z level).
If ramps were a bit more reliable/accurate (not just pathing devices) such that rocks that fell on ramps would then roll to their 'down' end, then I could see a chute collecting thrown rocks (by catapult or bridge) feeding another catapult.
Actually, it'd also be interesting to see "Indianna Jones"-style rolling rocks going down a multi-Z incline. But that'd mean making stones (or specially carved stone balls) fatal to things by movement other than the horizontal component of catapult chuckings. And dwarfs would have to be susceptible (if there was nowhere to get out of the way into) to make it balanced.