Not sure about now, but go through your closet and see what you can come up with.....
jeans, a plaid shirt, some boots....lumberjack
fancy dress, fancy hair, fancy makeup....princess/queen/fairy
shift dress and proper accessories...someone from the roaring 20's
Black t-shirt and some black garbage bags (or even better black tights) to make a spider. Make and stuff 8 arms and sew or pin to the black t-shirt
We have my kids black graduation gowns that we've used to be a graduate, harry potter (with the striped tie and glasses), a wizard (male), and a witch,
For future planning....go shopping a day or two after Halloween and pick up some accessories on clearance and start a costume bin. At this point in time we can create costumes from ages 4 to adult just by opening our boxes and seeing what tickles our fancy this year. Now some of our costumes are store bought and many are homemade but by mixing and matching we can come up with many, many creations. That said, the accessories are often the key so we have a number of wigs, hats, masks, etc. to use and when something gets lost or broken I keep my eyes out after the holiday for a replacement on 50-75% off.