I know its more like a myth than history, but what about "the lady of the lake" from the Arthuer legands? i did that when i was like 13? maybe. Wore a dress made out of sheer light blue fabric my mom got for a dollar a yard over a white tank top and skirt. It had blue ribbon trim and random pieces trailing. braided blue ribbons into my hair and left long ends hanging. used a light blueist lip gloss with blue eyeliner and that bout summed it up. I loved it. or more "history" oriented a fortune teller/gypsy?
hope they have a ton of fun! i love stuff like that, let me know what you come up with.