Some particular favorites of mine thus far:
How It Works:
I like reel challenges, but as far as I know they're supposed to be movie fusions only. I'd kinda like to include TV shows as well. We can have a list of shows/movies/etc and have people claim one as a prompt. Since a regular round here is a month, I think giving them three weeks to create something would be enough.
I think we can get a variety of works with something like this.
Accepted Fanworks: Anything you can come up with.
How It Works: Create a fanwork based on a song. It can be something from Kris or Adam's albums. It can be something they've covered. It can be a fic about how a theme week was different on Idol, and what songs they chose. The sky's the limit, you just have to involve a song.
Dates: (it would be cool to tie this with Music in Our Schools Month, but that's March, so I'll say open-ended)
Accepted Fanworks: ANYTHING. Write a fic. Create a fanmix. Embroider something with lyrics. Post a YouTube video of yourself dressed as on of the boys singing the song of your choice.