Defection
Action ~ Attack
$6
Reveal the top card of the player to your left. He may pay the cost of the card immediately, putting it into his hand. If he does not, then you may buy the card immediately, putting it on top of your deck.
Is your victim allowed to play Treasures immediately? Otherwise, they have nothing with which to pay. Even coin tokens would be useless here, because you can only cash them in at the start of your Buy phase.
One way to fix this is to let your opponent take debt tokens equal to its cost in coins. This would create weird interactions with debt and Potion cost cards, but it would eliminate most of the rules questions.
This was actually a modification of an idea already posted in the "do not do this kind of thing because it's too swingy" in the fan card guidelines post. So... I had to figure out how it could actually work. Thanks for the suggestions!
In answer to the comments and questions, yes, the opponent can play his treasures during your turn to "re-buy" the card. I feel like giving the opponent a chance to keep the card even as an attack helps with swingy-ness, but of course very few games will have the opponent have more than, say, $5 in treasure in his hand. And during your turn in most games, you could have enough treasure to buy anything by the end of your action phase. So, yeah, it favors the attacker a lot. That being said, this is a good card if you are losing, not winning, so I think it's not OP.
Still, here are a couple fixes. Let me know if one of these helps:
DefectionAction ~ Attack$6
Reveal the top card of the player to your left. He may immediately discard a card from his hand. If he does not or if the value of the card discarded does not equal or exceed the cost of the revealed card, then you may buy the card immediately, putting it on top of your deck.
DefectionAction ~ Attack$6
Reveal the top card of the player to your left. If it is an Action card, then he may reveal any number of treasure cards from his hand. If the total value of the treasure cards equals or exceeds the value of the revealed card, he may discard the card. If it is an Action card and he does not, then you may buy the card immediately, putting it on top of your deck.