I originally chose rust for 2 reasons... 1. There's plenty of rusty stuff around here, and 2. it's specific. Nothing drives creativity to the limit more than tight restrictions. Seriously! There's a billion ways of depicting emotion, and surrealism as well to those experienced with it, but what can you do with rust? You have to get creative, that's what! You have to come up with ways of incorporating this specific theme into images depicting a variety of styles, whether it be surrealistic or emotional or whatever you can conceive. I would also consider allowing for anything akin to rust, such as other forms of decay or man-made objects in states of disrepair.