I'm doing a witch. It's actually the first time I've done this costume. I'm still trying to work out what kind of look I want to use. I'm kind of leaning towards the more traditional look, though I'll probably forgo the makeup. I can do masks, but makeup drives me nuts.
I've also started looking for a black cat plush for a prop. A seller on Etsy offers a plush based on the black cat from Coraline. Really nicely done too. If I go with that, I may adapt the costume to fit the cat a little better.
Of course, with Witchy mentioning Steampunk, a Steampunk witch could be cool too.