It's probably something that requires a lot more thought... it'd probably take us the day just to come up with ideas and designs (preferably by somebody that knows how to design game-play).
I'd consider starting with something simple like the "predicting red or blue" card game focusing on calibration, mentioned here: Worse than random
So, the London community is arranging a Hackday where some of us will get together and code. In order to ensure we work on the awesomest idea(s) possible, we decided to ask LessWrong to add to our list of candidates. So here is the question:
What could a few developers do in a day or less worth of coding that will be awesome? Also, as a way of checking calibration, you can give your estimate for how long such a thing would take to build.
Note: While we will take ideas and voting here into account, there is no guarantee that we will actually end up choosing one or more of them.