Patron
ACTION $5
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+2 Coins
-1 Action; if you do, + 3 coins and +1 Buy. If not, +1 Action.
-It's either a Silver or a doubly-terminal Platinum with a Buy. I can bump it down a few ways, or bump the cost up to $6.
Might be too much. +$4 and +$1 buy is probably stupendous enough, especially since Silver is a pretty decent consolation prize. (Compare with Royal Seal and Stash, which are Silver-with-a-bonus cards. The bonus on this one is much much stronger, though admittedly not one you can always use.) But net +$5 seems crazy, even on something difficult to enable.
Another note on this is that it's impossible to activate on a good percentage of boards.
Portfolio
ACTION-DURATION $5
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Now and at the start of each of your subsequent turns: +$1.
This is strictly superior to Treasury, which has the same effect until you buy a Victory card, at which point Treasury gets reshuffled, while Portfolio stays out. So I think it needs a cost increase to $6, but it's probably fine at that price. I don't think TR/KC will be a problem, either: as you say, there's a significant downside to using them on this card that balances it out. I like how that works.
There are probably a lot of nice, simple cards of this sort you could make: Now, and at the start of each of your subsequent turns...
* +1 Buy
* +1 Action
* You may discard a card; if you do, +1 Card.
* Look at the bottom card of your deck. You may move it to the top of your deck.
* +1 Card. Put a card from your hand on top of your deck.
Tax Collector
ACTION-ATTACK $5
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Each other player with 4 or more cards in hand reveals 3 cards from his hand and discards one of them that you choose. If they discarded any Treasure cards, you gain a copy of each and put them into your hand.
-Not much to say, except maybe gaining each Treasure in hand is too strong? How about this: "If they discarded any Treasure cards, you gain a copy of one of them and put it into your hand."?
I made and playtested a card that's exactly like this except that the opponent revealed all but one card, rather than any three, and instead of the "If they discarded..." clause, the card offered a flat +$2. I think I'm happy with that card, although I want to playtest a version that requires 5+ cards to be vulnerable to attack, rather than merely 4+.
Despite the similarities, I think this would play very differently, mostly being weaker. The attack portion is weaker, because opponents will usually be able to save 2 cards rather than just 1. And the benefit to the player will
often be weaker, since (1) it would be unusual to ever get a Gold from this; (2) it would be common not to even get a Silver from this; and (3) there's a real danger, especially early on, of being forced to gain Coppers, as sometimes a player will reveal 3 Coppers from his hand.
In fact, on average the benefit to the player is probably weaker than Thief. It's hard to compare the success rates, since (1) with Tax Collector, the opponent gets to choose which cards are offered up as candidates for the attack, but (2) the opponent may well prefer to offer up good Treasure than risk a good Action card being discarded from his hand.
However, unless the success rate turns out to be dramatically better in playtesting, there is still the matter of Tax Collector hitting
weaker treasure cards. Additionally, the attacker
must gain copies of the Treasure instead of optionally gaining them.
All told, I think this needs some serious strengthening. Offhand, I'm not sure how to make a "gain a copy" effect work with this attack, since you'll frequently (perhaps usually) only have the opportunity to gain a junky card. You could try "You may gain a copy of any non-victory card discarded this way." That might work, but I still suspect the card won't be worth playing without a more certain benefit for the player. Like the +$2 I put on my own card.
[spoiler: this next card is more of a jokethought experiment than anything, but I'm curious if it is viable. I tried to make it so.]
Viceroy
ACTION-ATTACK-REACTION-TREASURE-VICTORY $5
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When played as an Action-Attack, each other player reveals his hand, discards one card that you choose, then draws 2 cards.
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When played as a Treasure, $1.
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When another player plays an Attack card, you may reveal and discard this from your hand. If you do, +2 Cards.
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1 VP
LOL. Excellent.
You've thought more about this than I have, so consider these thoughts as merely first impressions. But I think that having lots of weak things still adds up to a weak card. It's kind of a Dominion principle, that one good thing is better than two mediocre things. That's why one Gold is better than two Silvers, and Tactician is a great card. So my fear here is that even though this card does a lot, it's still not much more than a very marginal improvement on its strongest single effect. I think that makes this a $4, maybe even just a $3 card.