I think you should draw someone in their pajamas, a robe, like people are on christmas morning. No one in my family gets dressed til late in the day on christmas. Or a drunk woman in a sweater. Mom's like to hit the eggnog on christmas eve. You know, turkey's in the oven, shopping is finally over, in-laws are visiting... Or you know the crazy secretary at the office christmas party...
For halloween, draw an old man in a flannel shirt, sitting on a bench under a tree. My grandpa did that one year on halloween, letting all the kids think he was a scarecrow, then he'd scare the crap out of them.
Or, draw someone in a cashiers apron. Probably some young, unwed, teenage mother, who's working a second job over the holiday's to make some extra cash. You know the type the store's hire just for the season, who generally make the hell of shopping that much worse.
You could also draw some chick with huge boobs, and only enough of a top to press them together. That seems to be very common on halloween. If you draw the rest of the body, it has to include a skirt short enough to show her underwear. Costumes seem to just be a good excuse to be naked in public.
My final idea is to draw anyone you want, dressed up as some famous person or character. It can be a person or character that looks normal enough so as not to make your pictures too unlike your regular portraits. For instance, you could do, "Gale dressed as Xena" that would blend perfectly with your portfolio...