Grand chancellor. Put every player's deck into their discard pile, twice.
Haha. Great. Another dumb ideas:
Grand Counting House
Do this twice: Look through your discard pile, reveal any number of Copper cards from it, and put them into your hand.
Grand Treasure Map
Trash this and another two copies of Treasure Map from your hand. If you do trash three Treasure Maps, gain 10 Gold cards, putting them on top of your deck.
Grand Tournament
+1 Action
Each player may reveal two Provinces from his hand. If you do, discard them and gain all Prizes, putting it on top of your deck. If no-one else does, +1 Card +1 Coin.
So now some real ideas:
Grand Spice Merchant $5
You may trash a Treasure from your hand. If you do, choose one:
+3 Cards and +1 Action;
or +$3 and +1 Buy.
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You can't buy this card if you have any copper in play.
Grand Merchant Ship $6
Now and at the start of your next turn: +3 Coins.
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You can't buy this card if you have any copper in play.
And what about this? This would be powerful!
Grand City aka Metropolis $6
+1 Card
+2 Actions
If there are one or more empty Supply piles, +2 Cards. If there are two or more, +$2 and +1 Buy.
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You can't buy this card if you have any copper in play.