How about have a challenege to come up with a challenge? It would have to be themed, as all previous ones have been, author must also provide the solution, etc.   It might be difficult to put parameters around what a winning submission must be though i.e. must be challenging, but not absurdly so, etc.   On the bright side, one month would be the challenge to come up with a challenege, the next month, every tries to solve that winning entry - get two for the price of one :-)  anyways, just a thought.