I love the challenges too! It was one of Bella's picture challenges that spurred me to really start posting WIWs here. Based on my experience, I think the challenges are a really good and fun way for new members to get involved here!
Not sure if it's already been suggested, but maybe it might be a good idea to have a separate tab in the forum for challenges?
And, Angie, I don't mind in the least if you want to suggest challenges and not participate. It's all about being inspired anyway, and you always provide good inspiration! i like the idea of folding a challenge into a blog post, or having one inspired by a post (like an Angie's outfit post, or a Team X Team Y post or something).
ETA: Sometimes I make up my own challenges and I find them quite helpful in assessing my wardrobe. Like Wear All My Jackets, and Wear All My Boots. And didn't Tanya do No Denim For X Days?