What I would like is a standard way to add standard and non-standard weapons effects, not just weapons.
As for weapons, I'm thinking of:
Dwarven magnetized thingies (such as shields, or maybe big honking panels that rise from the ground for the more extravagant) that attract arrows and bolts and save your folk from becoming pincushiony little lumps of flesh. Of course, these won't work on bone, wood, and projectiles made of certain types of metal, so the enemy can still work around the dwarves having this in inventory.
Fortifications firing nets and other payloads, such as poison-filled barrels, gobbo body parts (for morale-lowering purposes--totems would get another use instead of just being something salable, and sewn images could conceivably be used for similar purposes, kinda like leaflet bombs. Ex.: *WE GOT BEER AND WE CAN WAIT WHILE YOU STARVE* or *YOU'RE NEVER GOING HOME, SQUEAKY*), and the like. Also, more accurate siege weapons.
The ability to make machicolations when 3D is implemented and pour boiling oil or lava on those pesky invaders, thieves, and salesmen.
More arrow types, such as bodkins and broadheads. Since this is a fantasy game, I don't suppose things like exploding arrows, sleep arrows, smoke arrows, flash arrows and the like would be out of line. In particular, I'd like gobbos to use flash arrows to blind cave-adapted dwarves, while dwarves can use stinky animal-based concoctions to render elves nauseous and incapable of defending themselves while melee fighters closed with them.
Something that temporarily turns allies against each other would also be interesting, as would artifact weapons and armor gaining specific abilities.