Allow me to start off by apologizing if this has been covered, but I couldn't find it anywhere.
If I had my way, blood would be added to the prestigious list of liquids, alongside magma and water. The current iteration of my idea would have 1/7 blood stored in a creature for every 1 size. Upon death, a dwarf would have blood spill out to a couple tiles beside them, while slaying a megabeast would be just a bit more "epic", as they would erupt into a pool of blood equivalent to 4 and 2/7 full tiles of water. It would make the "cleaning" labor somewhat useful, for people who want to upset the god of blood. At low depth, it would either evaporate, or dry out, I hadn't decided on that myself. This addition would give people a few extra things to do. I've thought of a few so that you might believe me:
*Building automated flushing methods for your Mortal Kombat style execution pits.
*Collecting as much blood as you can to show off to your fellow Dwarf Fortresseers.
*Filling your moat with the blood of your enemies. I know I wouldn't want to attack an adversary that had a moat filled with blood.
*Drowning people in blood.
*Filling a 20 level tube with kitten blood. (you can decide the usefulness of that later)
*Creating an enormous statue of a dwarf, use an elaborate pumping construct to make blood continuously pour out of his eyes/mouth.