It is a very nice idea, but I do not like the fact that you are able to submit predictions that were made too long ago - not to mention predictions that have already been proved or disproved.
It may encourage unnecessary bias in which predictions are submitted.
"It" refers to the fact that you can submit old predictions.
There is no doubt that a site like this will have some bias in which predictions are submitted. I do not doubt that it will be mostly extreme predictions that will be submitted.
But the fact that you can submit predictions, when you have considerably more knowledge (or even complete certainty) about the outcome than the predictor had when he made the prediction, seems to be a completely unnecessary source of bias.