I've noticed that a few dominion cards, rather than having a direct effect like +money or +actions, instead change the way another card behaves or the way a turn phase behaves. Let's call these cards "meta" cards. Coppersmith is the most prominent example: it makes copper produce an additional +$1. Hoard is a lesser example, since it changes your buy phase so you gain golds when you buy victory cards, as is goons, which allows you to gain +1 VP for buying things. Throne room and King's Court define their effects based on another card in hand, so they're also tentatively meta cards.
As you'll notice, not many meta cards exist and those that do are often quite powerful; goons and KC are unstoppable on the right board. Since the theme hasn't been explored much, I'm attempting to concoct an expansion around the concept, and I'm curious what other people think. Here are some examples:
Sewer
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+1 action
+1 buy
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While this is in play, you may set aside an estate or curse to draw 2 cards. At the end of your turn, discard the cards set aside.
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Reaction: When you gain a card, you may reveal this to trash it and gain an estate in hand.
Action/Reaction (5)
Effectively, this card turns an estate into a laboratory (wow!), so it might make flooding your deck with estates a viable strategy. However, any turn in such a deck that skips a sewer is going to suck. Note that any turn with a single sewer in hand in the initial deck will net $4 in the worst case scenario, and $5 if there is at least one estate in hand, so this card can make some big money.
As for the interaction with curses, it's probably too strong, but I wanted to include it just for thoughts.
Smelter
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+2 actions
+$1
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While at least one smelter is in play, when you trash a card, draw another card.
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Action (3)
This card is meant to support weaker trashers like trading post, which becomes +2 cards +$2 trash one card with smelter support, or trade route, which becomes significantly more attractive. I didn't want it to stack because, while drawing four cards for each card you chapel away sounds hilarious, it's probably broken.