Let's see, one of the females witches of the Harry Potter world meet a magical creature and either get turned or somehow involved with the creature. The witch is slowly drawn into the other's world and learn their way of life, perhaps either ending up fighting for Vampire or Werewolf rights or writing a book about the species of her new lover. The plot would be the two women coming to term with their differences and trying to make their budding attraction work.

OR, something more light hearted would be two Beauxbatons schoolgirls that are close friends slowly become more as their come into adulthood. Include mostly fluff with some drama with the entourage of both.