$5 -
? - Action-Reaction - At the start of Clean-up this turn, you may put a card from your hand on top of your deck. | When you gain a card, you may put it in your hand.
I was thinking of what design space has not been covered yet - we have Reactions that trash cards, put cards on top of your deck, give you silvers, increase your hand size, mess with the top of your deck, and block attacks, but nothing that would put a card directly into your hand. There are cards like Trading Post and Mine that, due to their nature, put a card in your hand, but wouldn't it be nice to Remodel or Workshop something and use it right away? Better yet, Ironworks it so you're sure you'll have the Action with which to play it. Imagine a turn where you go Village, Village, Bridge, Bridge, Ironworks - gain a Grand Market, and play it. Of course, you can't have just a Reaction card (well, I guess you could, but that would be kind of silly), so the Action would synergize with this Reaction. With two of these [unnamed] cards, you could play one, then use the Reaction of the other while in your buy phase to put it in your hand, then during clean-up put it on top of your deck, so you can use it next turn. Or simply use the Action as a delayed Courtyard. The problem with cards like Mandarin and Courtyard is that you have to decide then and there which card to put on top of your deck, and if you play more Actions, it may end up going right back into your hand - with this, you don't have to make a decision until the end of your turn. What do you think?