Well, I certainly enjoy issuing challenges and trying to ask questions to answer that call, but so far participation has been somewhat limited. Part of the reason might be that I'm coming up with the ideas myself, promoting the challenge myself, and I'm the only person who really has any ownership of the challenges.
So I'd like to include you in the overall planning of future challenges (and also gauge interest in the whole enterprise). Please answer this question with any ideas you have for future challenges. One challenge per answer would be best so that we can vote up good ideas (and, I suppose, vote down bad ones).