In the project suggestion thread OldSchool mentioned challenge projects for the monthly project (Good idea OS). Instead of using that for the monthly project, would you mind if a person would issue short term challenge projects in separate threads, much like folks sometime do in Viewfinder with "Post your [whatever] pics here" threads? The difference would be that the photos posted would have to be taken in response to the challenge project rather than something taken from one's archives.
I don't think they'd have to be 'stickied' or have a specified lifetime and would just naturally disappear when folks are no longer interested in them and perhaps pop back up once in a while when someone spots an appropriate subject.
If the subject line said "Challenge Project - [whatever]" everyone would know what it was about. Perhaps a sticky for a while that explained challenge projects would be good if you go along with the idea.