I'm a new Dwarf Fortress player, and while I don't have but a month or two's experience under my beard, methinks there's one system that could use an update.
Poisons! It's no fun to farm gnomesblight or any of those other fun toxins when they're so hard to get in your enemies!
I've figured the easiest way is through traps, especially Weapon traps.
Even broader, mabye be able to add plant extract to weapons, at least syndrome-causing ones. That's an easy way to poison your enemy! (Same idea with weapon traps.) Secondly, mabye a new trap you can load with barrels/flasks/bags of whatever gas-forming substance and once activated releases the pent up gases from the booze/extract oil/other inside the trap. I'm not sure what to do about temprature, mabye make the trap need coal and watch as the gas burns everything and poisons them to boot.
Secondly, seige engines. I was thinking about the Ballista, and thought "What if it were automatic?"
I realize this seems out of feeling, but an automatic giant crossbow is acutally feasible (ala Mythbusters.) Three parts and a mechanism, and the ability to link to a lever. Pull lever, however many arrows were loaded fire in rapid succession. Not sure how many it should hold.
Also, not sure what this is. A kind of door trap, that when opened, two sserrated disks swing out from the walls. Like a booby-trapped door.
Lastly, economics. This isn't so much a system than it is products. Why not have different groups of a civilization have different cultures? This one is just an idea, but if implemented, each culture builds, uses, and values things differently. Like in three different Human colonies, one might sell mostly weapons from all sorts of cultures (from longbows to katanas), one would sell all kinds of animal products, and one would sell semivaluable trinkets (that you could engrave and sell back, or something.)
The first human colony would be militaristic, attempt seiges, and value well crafted weaponry over much else.
The second would be willing to seige but wouldn't unless you piss them off, and value fine and luxurious meals and booze.
The third wouldn't have much of a military, and would value anything well crafted. (I.E., the multiplier for anything well made, engraved, w/e is doubled, average things sell for 0.5)
Of course, that's when everything matches up. Each culture has a favorite export item type, their favorite item, military strength, attitude (how quick to attack, anger, etc.) and other conditions that would guide their actions. And, for extra laughs fun, randomize it. You may end up with constantly angry weak-military chef people.