I was thinking about the barrels of blood, and had suddenly this idea. The longer I thought about it the more ridiculous it got, and therefore the more I liked it
I think this could turn out to be a great source of fun....
So, now we have this disease system and barrels of blood which can be bought. A dwarf can become infected with vampirism under certain circumstances. At first it is nothing bad and can be cured, but after some time the dwarf dies and comes back as a vampire. He is stronger, faster and more difficult to kill, but has also a few drawbacks, like, spontaneous combustion when outside. (Shouldn't kill him yet, so that he has a chance to run back into the fortress and spread fire). Also, instead of a bedroom he now needs a tomb to sleep in, and drinks blood from barrels instead of eating (he might still drink alcohol, though). If there is no blood, he has unhappy thoughts, and will at some point attack and suck dry at first pets, then other dwarves.
Also, his friends and relatives will get unhappy thoughts about him becoming a vampire, dependant on their preferences more or less than if he had just died.
He himself should not be too happy about his state, too. Maybe it gets better when he gets buried before he comes back, maybe dependant of the quality of the tomb. So, if you are lucky you get quite a powerful dwarf, and if not, you will have a dangerous monster running berserk in your fortress.