I think the best forum games are those which encourage creativity in the participants. Therefore, my submission is a game in which you give people a limited amount of time in which to invent their own game. It would have to start at a specific time, described in the OP, so that people who see the thread later due to timezones or being busy for one day don't get disadvantaged, and it'd have to have a specific cut-off time due to timezones, etc. For the same sorts of reasons, a decent time period should be given to potential participants so as not to dissuade people from playing due to short time constraints, if they have commitments or some such. 

How's that for meta, David? :rolleyes

Nobutsrsly, this is a really cool idea. The comments above are just my roundabout way of giving constructive criticism. If I have time I might try to hash out the rules for a game.