A free for all is good in that you get a lot of folks who can take what they are working on and direct it to a competition. I think you might want to set up a few things - like saying the game has to have a start screen, a "score" of some kind, and an ending -- basic things like that instead of saying "it has to be a space shooter".
If you have a theme, try to do something like "rebirth" or "seasons" -- something folks can take and run with.
Don't just have 1 winner - folks may give up if they thing they would only come in 2nd or 3rd. Perhaps having winners like "best use of theme" and "best overall" but also "people's choice" perhaps...