Okay, first - question: does TRC canon ever say anything about turned vampire blood's abilities to heal others? I'm a bad CLAMP fan who doesn't remember.
Now, plotting:
Sebastian is a turned vampire who once took Ciel in out of sheer boredom when he found the poor kid as the sole survivor of a vampire attack. They've travelled to town in the hopes of a) finding and killing the pureblood responsible for murdering Ciel's family and b) finding a pureblood to turn Ciel as well as c) explore the possibility of vampires being blood-bonded. For. no other reason than them being messed up people who don't feel right if there's no creepy magical interdependent bond between them.
If people are okay with her having a leading position in this AU too, I was thinking Elsa could be the human mayor of the town, oh so stressed about all these attacks and the safety of the townspeople. The thing is, she's oddly reluctant to take any big measures to stop the attacks and is acting kind of cagey about the whole thing - the reason being that her bodyguard, Bucky, is really a vampire, and she's concerned about his safety.