I dont know I feel like instead only pushing skills in modeling, rendering or lighting by actually giving static topics it would be maybe interesting to also try to encourage people to their creativity.
Why not just giving them a single word like "SPACE", which could be anything from the tiny narrow mousehole to the big universe. I think that would be a great alternative to the standart :" model a car or model a house " contests, which are of course also great in order to learn the tools. But Im somehow missing the creative part in the contest, at least in "smaller" communities and battles. Just a thought.