What about a challenge that simulates a job in the industry. I've never worked in the industry so I could be wrong. It seems to me that typically someone sketches a design or concept. Once it's approved, then it goes to the modelers to model, then to the texture artists, then the lighting people. So what about a similar challenge based on a certain concept. Could even be an image really.
If it's a modeling challenge, then whoever's model best portrays the concept wins. Can do the same for texturing. As far as lighting, perhaps the contestants have to follow specific guidelines to follow like making the image look like a moonlit night, or hot summer day, or romantic indoor lighting, or an overcast evening, or perhaps the sun is setting, etc.
I know it doesn't allow for any creativity because everyone would be working on the exact same thing but the thought is that it would simulate what artists face on a day to day basis in the industry. Again, I'm just guessing here.
Don't be satisfied with what you can do but rather strive to do the things you can't do!