|Might be cool to have a challenge in which a particular kind of "substance" is a major element.
By "substance", I mean having a challenge in which something like: concrete, asphalt, rubber, metal, fabric, ice, leather, plastic, rubber, or rock is chosen to be the major element.
For example, "Take a photograph in which concrete is a major element."
Could make it more slightly challenging by requiring more than one substance in the photograph... (ie. concrete + rubber or ice + fabric)