Okay, sorry I had to leave so I didn't explain my post very well. I mean a playtester posts one card that they ARE making in the next set; yes, it gets spoiled. But then everyone on the board takes their time to post ONE good, well-thought out idea of what they believe the card should look like. Then the best idea, or a close variation, takes the cake and gets made into a card. I think that'd be a cool idea.
For example, we know that some of the old heroes like Peter, Philip, etc. are going to be reprinted eventually, same with Goliath for evil. So the playtesters commit to making a Philip card for example, and everyone posts their best Philip. Playtesters take the ideas and come up with the most awesome Philip. I think it could be effective. They could do this for 1-3 cards per set.