I think a single species contest would be awesome, but I'm not sure how many people would be up for it. I would
Why not have a trial run? Polls are nice, but when you really get down to it, the number of people who decide to participate will determine success.
I think another neat idea for a contest is a journal type thing, and I'd be willing to participate in this. The participants would start with an empty tank (size of their choice), and light, filter, substrate, CO2, etc would be totally their choice, and plant selection, 'scaping, etc, would be their choice. This might a be a little tougher on the wallet, or us "cheapos" can show everyone how we get by with less.
The contest would start out with pics of the empty (equipped) tank and then the tank newly planted. A weekly photo submission keeps you in the contest after that. 9-12 weeks would be a good contest deadline.
This type of contest would be more educational to new people and experienced alike for obvious reasons. Further, it could be a chance for the low-tech and non-CO2 people to show off. Of course, 9-12 weeks might be a little limiting for them.
Lots of people start these journal types of threads in the forums, but few, including myself, follow through for various reasons. Incentive might keep us on top...