This may be a little too kingmaking-y for your tastes, but its an idea I've been thinking about since my first dominion game: having next player / previous player interaction. I submitted the first two cards to a contest previously.
Enclave
Victory
1 VP
Worth 2 VP for the player to your right.
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When you buy this, the player to your left gains it instead.
Cost 5$
Informant
+1 Action, +1 Card, +1$
The player to your right draws a card.
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When you buy this, the player to your left gains it in his hand instead.
Cost 4$
Charity (was named alms, but then adventures happened)
Treasure
1$
The player to your right may discard a Treasure card to draw a card.
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When you buy this, the player to your left gains it instead.
Cost 3$
Defectors
Action
+1 Card, +2 Actions
Trash this card. The player to your left gains it.
Cost 0$
Project
Action
Trash a card in your hand. Each other player may gain a copy of that card. Gain a card costing up to 2$ more than the trashed card.
Cost 3$
Negotiation
Action - Reaction
+2$, +1 Buy
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When you gain a card in another player's turn, you may discard this. If you do, choose not to gain that card, or to gain an extra copy.
Cost 4$
Too much kingmaking? Too little design space? What does f.ds think?