What I'd like to see is some massive Heath-Robinson device-cum-invader trap, which spans most, if not all, of the entire fortress, involving computation of water levels, magma, supports, pressure plates, prisoners, and kittens with an overarching purpose of doing something ridiculously simple (like activating an atom smasher, or dropping a bridge, or something).
Now that would be unreasonably complicated.
Yeah, like you pull the lever and it starts a big of magma flowing. The magma hits a goblin prisoner whose clothes catch fire and he runs towards the water area. His fire turns some of the water into steam and this water change activates a pressure plate that triggers a support which blocks thousands of tons of rocks 20 z levels and a slowly filling water room. The mist knocks out a kitten who tumbles from her platform down onto a bridgapult while the water room finally fills on a pressure plate that triggers the firing of the bridgeapult. The kitten flies to a burning lignite box and runs to a booze stockpile, which explodes. The burning booze kills an entire family and drives the father insane who pulls the lever while insane to close the floodgate the siegers are using to enter the fort.