Here's a few cards I've been working on trying to make the idea work. Most of these are still untested so far.
Stablehand
$2 - Action - Reaction
+1 Coffers.
Choose one: +1 Action; +1 Buy; Trash a card from your hand; or Gain a Horse.
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When you trash a Treasure, you may play this from your hand.
This originally costed $3 (and was called "attendant"), but someone said it was too weak so I made it cost $2. I think the game could use some more decent $2 Actions anyway.
Lumbermill
$4 - Action
+$2, +1 Buy.
Choose one: Add a token to your Lumbermill mat. Or remove all tokens from your Lumbermill mat for $1 per 2 tokens removed, then trash this.
(or $1 per token, but that seems like it might be OP)
Lumbermill B
$4 - Action - Duration
+$2, +1 Buy.
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At the start of your buy phases while this is in play, choose one: Add a token to this; or remove the tokens and discard this for +1 Buy and +$1 per token on this (but not more than +$6).
I don't like Farmer's Market, so these are attempts at something similar that doesn't screw over the other player.
Buttinski
$4 - Action - Duration - Attack
You may trash a card from your hand.
Now and at the start of next turn: +$1.
Until your next turn, the first time each other player trashes a card costing $2 or less on their turn, they discard the trashed card.
Originally, this gave Silvers instead of Coppers but that didn't seem like much of an attack.
The bad effect happens to you as well on the turn this is played. The wording seems a little weird... [EDIT: I reworded the attack and made it only trash once.]
Gemcutter
$4 - Action - Duration
Trash a treasure from your hand. Gain a Treasure costing up to $3 more than it, putting it on your deck.
At the start of your next turn, discard a card, then draw up to 6 cards in hand.
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When you gain this, play it.
From a recent weekly card design contest, this originally cost $5 but was too weak.
Jinx
$5 - Action - Attack - Duration
This turn, when you gain a card, put it under this. Gain a card costing up to $4.
At the start of next turn: shuffle the cards under this into your deck.
Until then, when another player ends their turn without gaining a copy of a card you have set aside, they gain a Curse.
An attack that's kindof the opposite of Smugglers. I don't like how wordy it is. I've tested this one a couple times and it seems to work. Originally, it put the cards under it into your hand at the start of next turn, but that was too strong. Maybe using a fan mechanic like the Queue mat would cut some of the wordiness.
Golden Flute
$3 - Action
If you didn't spend an Action to play this, +1 Card.
You may discard a card to replay the previous Action card you played this turn if it's still in play. If you don't, +1 Action and +$1.
I have several cards with the "If you didn't spend an Action to play this" theme. I've tested this in a few games, and I'm thinking of changing to cost to $2. (This was also submitted to a weekly design contest)