I'm not driven by competition, but from the sidelines let me suggest [not in order of importance]:
1. Set a time period for each round -- 2 weeks, 1 month, etc.
2. State a specific 'challenge' for each round. This could be a type of picture [calendar, fashion, portrait, sports, children] or a picture defined by mood [sorrow, anger, fear, love] or by a specific subject [tree, car, camera, flower] or -- you get the idea.
3. Develop a simple but useable list of criteria for judging. Suggested categories include adherence to the specific challenge, composition and originality. [Do be careful here -- sharpness and precision of image may penalize those who are not able to afford $$$$$ gear. Same goes for pics using extreme lenses, etc.] Make sure the judges are in agreement with the criteria. A 1-5 in each category should do the job. Consider whether the contestants should know the criteria or not.
4. You will not know in advance how many will enter the competition. Set up the elimination so that the first round or two cuts the field sufficiently to ensure that the competition will not go on forever.
5. As soon as possible, set up a working committee among yourselves to get this idea structured and firmed up. One of you should speak up or be selected as the committee chairperson. You might want to do this through email or PM's rather than the open Forum. Saves space on the open Forum. Saves face if it bombs.
6. If this concept starts to take shape, contact a moderator soonest for advice.
Side notes:
I don't know what you can do to ensure that the photos submitted will be taken during the challenge period and aren't shots taken years ago from a deep library. Perhaps all you can ask is that the contestants follow the 'honor' system.
It might be fun to have one round require a throwaway as the camera. Talk about levelling the gear field!
Do you want a prize for the winner other than a round of applause? You're asking folks to invest quite a bit of time and effort to compete.
Amateur/pro categories?
The idea sounds great. Hope you can get it rolling. It's my guess that there's a Santino or Jay or Sara out there somewhere. While I will not compete, and haven't the credentials to judge, I'll help if I can.