The main thing missing, I think, are the features to let all this be done by community practice and less by the site authors. Wikipedia practices like NPOV and voting subject to oversight by admins, and something like a Wikiable paragraph for each prediction entered. And voting on the most interesting predictions (so that the site can have a most interesting prediction of the day up front). If I were going into this full throttle, those are the preliminary features I'd be looking for.

It actually seems like something that ought to be part of Wikipedia, but unfortunately Wikipedia seems to have become the de facto standard while maintaining a very closed practice that limits how much information it can accumulate and whether it can have special interfaces for particularly regular sorts of facts.