My costume idea is a fury. This is really easy, cause you have lots of different possibilities.
Either get a brown short dress (it shouldn't be the most beautiful) or a dark red top and a dark red half long skirt (colors are not that important, you can also use a brown skirt, just keep the colors dark). Wear boots or comfortable brown/red shoes but don't wear high heels.
Your hair should be very, very untidy. Now use some make up, your eyes should be dark. Use a kohl pencil to draw dark flames-like stripes all over your face, just be creative.
You can use some dark-long-fake finger nails too (optional) and try to fix a few leaves in your hair (optional)
Thanks to Rosi for this different idea!
Note: Furies are three daughters of Nyx (Night); Alekto (Alecto), Megaera and Tisiphone in Greek Mythology
Fury: Erinys; used in the singular to denote an avenging goddess; some translators refer to her as Strife.
Source: Stewart, Michael. "People, Places & Things: Fury", Greek Mythology: From the Iliad to the Fall of the Last Tyrant. http://messagenet/myths/ppt/Fury_1.html