I have to admit it, I'm kind of in love with Graphic Nothing's (Gary Clarke's) Movie posters for Minimalists. They remind me of those flimsy classic paperbacks you used to see in bookstores for a dollar (do they still sell those? I'm really out of touch with real world bookstores now).
The vague rules were a circle, lowercase text & one or two images (not from the film) that relate to something in the film (which will not probably make sense unless you've seen the film!)
Sounds like a great challenge, and you could easily swap out those rules for other similar limitations.