I have been a member on CGTalk for a while and they have competitions. What they mainly do is have the admins and mods set the competitions and in their own sections and set a close date depending on how long they think it will take. If people have their own ideas of course there can be a section in the competition section that ppl can post their ideas and then the admins read through them and pick which one to do maybe have a couple running at once or just one.

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