This started out as a joke, then a thought experiment, then a challenge. None of these have been thought through all that much, but I think it threw out some interesting ideas.
Anti-Loan
Treasure - $4
$1
When you play this, reveal cards from your deck until you reveal a Treasure. Gain a copy of it. Discard all the revealed cards.
Anti-Trade-Route
Action - $3
+1 Buy
Gain an estate.
If you do, +$4 and reveal your hand. -$1 per victory card in your hand, to a minimum of $0.
Anti-Watchtower
Action - $2
Choose one: Trash a card, or put a card from your hand on top of your deck.
When you discard a card other than during cleanup, you may set this card aside. If you do, draw until you have 6 cards in hand, then discard this card.
Clarification: If a card asks you to ‘discard down to X cards’, you discard them one at a time and may react after discarding any of them. If you react to the last one, you have already discarded down to X, so you draw up to 6 and stop. Otherwise, you draw up to 6, then discard cards until you have X cards in hand.
Anti-Bishop
Action - Attack - $5
Discard a treasure. If you did, name a card in the supply.
Gain a number of curses equal to one plus half its cost in coins, rounded down.
If you gained at least one curse in this way, gain the named card.
Each other player gains a curse.
Anti-Monument
Action - $3
+$4. Gain a curse.
Anti-Quarry
Treasure - $4
Worth $3. While this is in play, Action cards cost $2 more.
Anti-Talisman
Treasure - $4
Worth $3. While this is in play, you may not buy cards costing $4 or less.
Anti-Worker’s-Village
Action - $2
+1 Card
+1 Action
+2 Buys
Anti-City
Action - $5
+1 Card
+1 Action
If there are no empty supply piles, +$1, +1 Buy
If there is up to one empty supply pile, +1 Action
Anti-Contraband
Treasure - $3
Worth $1
+1 Buy
When you play this, the player to your left names a card from the supply costing at least $3. While this is in play, that card costs $2 less.
Anti-Counting-House
Action - $4
Discard any number of coppers. For each copper you discarded, +1 Card, +$1
Anti-Mint
Action - $4
Trash a treasure card. If you did, gain a gold.
When you buy this, gain two coppers.
Anti-Mountebank
Action - Attack - $5
+1 Buy
Trash a treasure. If you do, +2VP.
Each other player may discard a treasure. If they don’t, they must return a VP token to the supply.
Anti-Rabble
Action - Attack - $5
Discard down to three cards. If you discarded any cards, each other player reveals their hand and discards a treasure or action of your choice, then draws up to five cards.
Anti-Royal-Seal
Treasure - $4
Worth $2
While this is in play, when you gain a card, you may discard the top card of your deck.
Anti-Vault
Action - $5
+2 Cards
+$2
Spend any number of $. Draw a card for each $1 you spent in this way.
Each other player may draw a card. If they do, they discard down to four cards.
Anti-Venture
Treasure - $5
Worth $3
When you play this, reveal cards from your deck until you reveal a card that is not an action or a Treasure. Put that card on top of your deck. Discard the other cards.
Anti-Goons
Action - $6
+1 Buy
Each other player gains a curse. If they do, they draw up to 6 cards.
Discard a treasure. If you do, at the end of your next buy phase, +1VP per unspent buy.
Anti-Grand-Market
Action - $3
+1 Card
+1 Action
+1 Buy
You can’t buy this unless you have a copper in play.
Anti-Hoard
Treasure - $5
Worth $2
While this is in play, when you buy a Treasure card, if it costs $5 or more, gain a Duchy; otherwise, gain an Estate.
Anti-Bank
Treasure - $5
When you play this, it’s worth $4 minus $1 per Treasure card you have in play (counting this).
Anti-Expand
Action - $2
Trash a card from your hand. Gain a card costing up to $1 more than the trashed card.
Anti-Forge
Action - $2
Trash a card from your hand. Gain any number of cards whose total cost equals the cost of the trashed card.
Anti-Kings-Court
Action - $2
Choose up to three Action cards in your hand. Play them in any order.
Anti-Peddler
Action - $8
+1 Card
+1 Action
+1 Card
During your Buy phase, this costs $2 less per unused Action.
Soil
Treasure - $0
Worth $0
When you buy this, +$1
Mud hut
Victory - $0
Worth 0VP
When you buy this, +$1