Let's see. Long black dress to act as a sort of shift, dark green overdress with gold ribbon trim, black hooded cloak with green stones and black velvet-ish material as trim. I have green suede boots that I might use, as well. I plan to wear a platinum blond wig (and hide my really dark hair under it) and carry a silver mirror as accessory.

So what if it's overkill? It should be a lot of fun anyway, and there's even a costume contest, so why not? ;)