Tetris would actually be pretty awesome.
Seeing as how we tote this community as a locus for game development, I'm surprised that there's nothing on there about... y'know.... making games. A short game design competition would be welcome, I figure.
Well, there's the mapping event. I think it's better to go with mapping because it takes less time than to make a game and there was a game design competition pretty recently. Short game in one map could work though.
I'm definitely going to take part in Soldat and hoping to take part in some other events too. SSBB and maybe mapping, TF2 and speedrunning although I suck in TF2.

And Tetris if we get that.