My idea would be to have a contest on a weekly theme. The entries would not be shown on the front page but on a separate page where other people can vote/comment. Only the winning submission would then be promoted to the front page.
I like the idea of a theme, just to get inspired. And it is always cool to see what others have in mind compare to our own ideas. Maybe it could be there, and only people who wants to do it does it, and others just simply continues posting what they want.