Like someone else mentioned, basically the net is swamped with challenges! Blogs, other forums, you name it. I even tried a weekly Monday Mania Challenge on my blog. Today was my 9th one, and the whole time I only had 2 takers. I doubt I'll take the time to come up with any more - there's just toooo many other bloggers out there doing the same type of thing. And me personally - I spend too much time surfing and not enough time stamping. I used to be a challenge junkie (like sketches and color combos the best) but I rarely do them anymore. Mostly lack of mojo here lately though.
As for this site - one thing that I used to take part in on another forum that eventually died is virtual stamping parties in the chat room. It was a blast - we had games, virtual stamping challenges, trivia contests, scavengar hunts in the gallery. You could do a morning one and an evening one. Maybe once/month or every other month. You'd need 2-3 hosts to keep it running smoothly. And started a new game/challenge every 15 minutes. They're very fast paced and a lot of fun!