A vaporware competition might be fun. See who can create the most hype for a game that doesn't exist.
Man, I could imagine that would be a frustrating competition to watch happening. Looking at all the awesome ideas, complete with fake screen-shots photoshopped together and knowing that they're not going to be made.
Then finally voting for the winner occurs. Every comment on the voting thread is "Here's my vote, you cold, heartless bastard that didn't make [insert awesome game name here]"
Although perhaps a couple people would look at their ideas and begin thinking about them, and realize that they are doable. And suddenly a series of amazing indie games would begin coming out because the designers hadn't thought about how hard the game would be to design when they conceived of the idea. The sky had been the limit. They had raised the bar as high as they could make it, then reached for it.
And a new golden era of indie games would begin.
Either that or people would come up with a bunch of ridiculous joke game ideas.
Either way, I'm giving my vote to "High Powered Wild Animal Cannon Vs. The Frogipult 2010"