Dwarf Fortress presently features fairies, but they seem to be just decoration at the moment. They aren't trappable, don't gnaw food, and aren't hated by dwarves, so they can basically be ignored.
One idea I had -- In a certain popular adventure game series, fairies are powerful healing items. Nintendo didn't specify exactly how they were used, but VGCats has a rather dwarfy theory --
http://www.vgcats.com/comics/?strip_id=118.
How it would work is simple. First, a way needs to be added to bottle them -- I assume [VERMIN_NOTRAP] means that neither set animal traps nor the "Capture Live Land Animal" task will work. Then we just add a new command to order the dwarves to use one on a specific wounded creature.
When used, the fairy would vanish, and each part of the patient's body would be healed one step -- even if healing would normally be impossible (amputations and brain).
We'll probably want the elves to trade in fairies, so that dwarves who get injured in Neutral or Evil climes still have hope.
It's a little silly, but it would provide a way to salvage permanently bedridden dwarves, and more importantly, would allow the player to exercise more sadism.