I know there was already a 10g nano contest, but I think it would be cool to see a contest for the least amount spent on a tank (retail value on drsfostersmith, liveaquaria, etc) that supports at least 3 coral and 3 fish for at least 6 months. We wouldn't count supplements/food/maintenance costs.
Another option I think would be cool is just a spin on that. It's the 10g nano, but you get graded on the appearance of the tank and get bonus points if you go under, let's say, a $300 equipment and fauna/flora budget. You can't go over that budget. Prices on equipment and fauna/flora is based on drsfostersmith and liveaquaria.