This is the most interesting and innovative card I think I've ever thought up
Pin
Cost: 5(?)
Reaction-Attack-Duration
Whenever an action card is played, you may reveal this from your hand. If you do, place this on top of that action card. Both of these cards remain in play until the end of the game. (At the end of the game, all cards go back into their owners decks).
Basically, you pin a card to the board, and it stays in play all the time. There's probably some rules clarifications or rewordings or something that need to be made. The most common use, I expect, is to just take a card out of your opponent's deck. I pin your KC, you can't use it anymore. But it also has some interesting interactions with durations (maybe?) and 'while in play' cards like highway, goons, etc. Pin your own goons, and then every card you gain for the rest of the game gives you a VP chip. Pin a highway, and everything is permanently cheaper (but for everyone!)