It was recently mentioned that it would be fun to have challenges posted from other people, and I agree! It's somehow less fun to take part in a challenge that you posted, so I'm keen on this idea for selfish reasons. Also, then I don't have to think of ideas. Win Win.
So, if you have a specific idea for a challenge, please email me (firstname.lastname@example.org) the text to post along with any images that are needed. I will then post the challenge at an appropriate time.
Also, if you consider yourself a designer I will accept some challenges that come with a special freebie to get started (a layout template, etc). If you're interested in this, just email along the freebie you'd like to include.
Looking forward to what you come up with!