What a terrible experiment. What about this one?: Look at these cards (rinkworks's mini-set design contest winners).
Sorry to disappoint you! My experiment had the feature of only making one guy who isn't here feel bad. Probably you guys can do better, but, well.
Have I told the Richard Garfield story? Richard playtested for Seaside. At one point he randomly suggested a few new cards. They were: 1) the treasure throne room, which I had already tried but later abandoned and then even later fixed up as Counterfeit; 2) Bank, again already in a set; and 3) a reaction that punishes the attacker, which I'd already written up an essay on so I could just show that to people who suggested it. Treasure Throne Room and Bank were both good ideas, but they were obvious and I had a big head start. So... Richard Garfield, three for three.
You guys have an edge here; you know not to make cards from Prosperity and so on. I have had even more years to pile stuff up though. So, say, a victory card that varies in value based on whether or not the Provinces sold out, that one I tried in a couple games with a proxy but never bothered printing out.