Most of the things I have thought of as suggestions have already been suggested elsewhere but I thought of something today which I think is a bit different.
I've got in the habit of assigning a few dorfs to do nothing but refuse collection. It occured to me though wouldn't it be cool if there was a type of creature you could tame and breed that went around your fort eating garbage in the same way that cats run around eating vermin? This trait could make them a very valuable resource, perhaps they only occur in certain environments, most likely subterranean. Possibilites include some kind of blind, subterranean mole like creature, some kind of worm like creature or even a sentient gelatinous blob or cube. Perhaps the untamed versions of these could even provide a challenge by being something of a nuisance, eating your valuable goods and the occasional unfortunate dorf for his tasty clothing and trinkets. Extending the idea further perhaps there could be creatures that eat stone, metal or gems? Even including objects made of those materials. Tame versions could be kept for disposing of unwanted stone or other material surplus. They could even provide an amusing way to dispose of captured goblins - ceremoniously wrap a goblin in fine armour and jewelry then release him into a pit as an offering to the mighty Cube! Then your dorfs can watch and laugh as he gets slowly dissolved inside a semi-transparent goop.
Some other ideas I had that I'd like to see or have already been mentioned I would just like to add my vote for:
-temporary scaffolding
-bird/reptilian livestock & egg production as food source
-beasts of burden useable to pull wagons and ploughs
-donkeys and horses able to breed to produce mules
-wagons and barges for transporting large quantities of resources over great distance
-ploughs to help speed up farming
-beasts of burden can be assigned to a quernstone to operate it and can operate other mechanisms where watermills and windmills cannot be built.
-possibility to produce railway system for mining carts - movement of resources
-ability to assign animal only or dorf only meeting points
-manufacture of gunpowder - the game already contains saltpeter, one of the ingredients of gunpowder; perhaps gunpowder could be used for advanced siege engines (cannons and bombards), arquebus' for dorfs and most importantly, rapid but dangerous mining.
-oil could be extraced from the earth as a resource and used for a variety of purposes; most notably flammable throwing pots and other incendiary ammo, and of course siege defense where lava is absent.
- ability for dorfs to construct Golems
I'm sure I had other ideas but I've temporarily forgotten them. Might post some more in here later as and when I remember.