I'm in agreement with the "more open" thing, I prefer having less constraints on what I do.
The two word type challenge that Tweety mentioned sounds great to me as I wasn't a member when that was going on, and it does leave a lot of room for ideas, whilst still having an aim to it.
Not too sure about a free for all, while that seems like a great idea in theory, I think the less experienced members may get put off joining by the more experienced ones really going for it, I'm imagining that happening as people will love being able to concentrate on their own area of interest in 3D, the car people creating amazing cars, character people creating superb zb sculpts etc.
And really, how much of a headache would it be to judge when all the final entries are so different?!

I think having something with a very loose theme gives the most creative freedom, while still having a point to the whole challenge.

Just my opinion, I've got free time at the moment so whatever the challenge I'll probably give it a go anyway....lol