Supernatural/Pride and Prejudice, although with only P & P characters and OCs (and, I guess some of the longer-lived Supernatural characters, just not any of the humans). Set in/around P & P times. The reflex might be to do it as an AU, but I think I personally prefer it not to be.

Elizabeth Bennett (or Darcy) is a BAMF Hunter, eventually almost a legend, despite...well, everything. Mostly gender. How did she get into Hunting? How did she get to be so good? How do her interactions with other Hunters go? What's her life like?