Wow, thanks for the fantastic feedback! I think most of your criticisms are spot on. I'm going to suggest some changes for some cards and push back on some others, explaining my reasoning for why I made them the way I did. If you've got time, let me know what you think.
Missionary
Types: Action – Reaction
Cost: 2
+1 Coin. Draw until you have 5 cards in hand.
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When another player plays an Attack card, you may reveal this from your hand. If you do, trash down to 3 cards in your hand.
I think you actually like this card better than I do. The way I see it, it's got two problems. First, as you said, it may be too powerful when you are attacked, allowing you to trash some junk and then restore your hand. Second, I think it's not powerful enough when you're not hit by an attack.
Here's my proposed change:
MissionaryTypes: Action – Reaction
Cost: 2
+1 Coin. Reveal the top two cards of your deck. Discard one of them and put the other one into your hand.
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When another player plays an Attack card, you may reveal this from your hand. If you do, trash down to 3 cards in your hand.
This makes the card less powerful after you've used the Reaction ability: you can't just restore your hand to five cards. As a bonus, it's also effective against deck-rearranging attacks, which the Reaction ability doesn't really help against. I've always liked that about cards like Secret Chamber, Watchtower, and Horse Traders. Their action ability helps against most of the attack effects that their reaction ability misses.
As an added note, the reason that I really wanted to include this Reaction ability in this set is that it allows the card to trash itself if you think you've gotten enough use out of it. That's kind of like a one-shot, right?
Tinker
Types: Action
Cost: 3
Reveal the top card of your deck. Choose one: Trash the revealed card and gain a card costing exactly 1 Coin less than it, putting it on top of your deck; or trash the revealed card and gain a card costing up to 2 Coins more than it, then trash this card. +1 Card. +1 Action.
First of all, I like your rewording, so I'll go with that. The reason I didn't word it that way in the first place is that I thought I'd need to start the card with, "Trash the top card of your deck. You may trash this card." This creates an ambiguity when you later refer to 'the trashed card'. Your 'reveal then trash' wording nicely circumvents that issue.
As for the card's actual effect, I admit that it's a gamble. There's a lot of luck involved. Overall, though, I think the effect is a net postive and a little luck can be a lot of fun sometimes. Let's walk through what could happen when you play Tinker. Chances are good you're going to open with it to maximize the number of Coppers and Estates you can trash with it, so let's assume that.
Scenario #1: You reveal a Copper, Curse, or Estate. Obviously you're usually going to pick the '$1 less' option and trash these cards. In this case, Tinker is comparable to Spice Merchant's cards/actions option, except that it allows you to trash Estates and Curses as well as Coppers. It trashes a bad card, draws you a card to replace itself, and gives you an action. It doesn't cycle quite as much as Merchant, but that's not that big a deal. Overall, success!
Scenario #2: You reveal a $2 card that you bought. This will almost always be bad. It's not too unreasonable to just say, "Maybe I won't buy $2 cards if I'm using Tinker." For a lot of $2 cards, that's probably a good tradeoff.
Scenario #3: You reveal another $3 card, like Silver. Chances are good that you don't mind trashing that card in order to gain a juicy $5 card. This may seem expensive, since you lose both your Tinker and your original $3 card (which you didn't get to play), but remember that Tinker still draws you back up to 5 cards, so your buy this turn is likely to still be worth at least $3 or $4, maybe $5 or more depending on what stage of the game you're at. Overall, this isn't as great a result as turning up a bad card, but it likely improves your deck overall at the cost of not being able to play that $3 card this shuffle.
Scenario #4: You reveal a $4 card. This is actually a pretty good pull. You could just take the Gold or good $5-6 card and buy a decent card this turn, just like in Scenario #3. However, if there's a nice $3 that you really want to play this turn, you can gain that, putting it on your deck, and immediately draw and play it thanks to Tinker's +1 Card/+1 Action.
Scenario #5: You reveal a $5 card. You just won the pain lottery. This is probably the single worst result for most games. Unless there's a $4 card you really want to play right now, you lose your Tinker to either replace the $5 card with a copy of itself or a Gold. Either way, you don't get to play that $5 card this shuffle.
Scenario #6: You reveal a $6 card. If it's early game, grab a good $5 coster and play it immediately. Otherwise, gain the Province.
Scenario #7: You reveal a Province. Take another one and trash your Tinker.
Things get a little wackier with Prosperity cards, but that's what you're looking at most games. Really, the $5 pull is the only catastrophic result. Even that can be OK if there's a $4 card that you really want to play this turn. That's one of the things I like about the card, though. Like Swindler, its power varies greatly depending on the distribution of cards at each price point, and those dynamics may not be immediately obvious.
Prospector
Types: Action
Cost: 3
+2 Coins.
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If this is in play at the start of Clean-up, you may reveal a Duchy from your hand. If you do, trash this and gain a Gold.
I completely agree that this card needs work. It was added to the set very late because I wanted a $3 card that generated $2, or did something roughly equivalent. I felt like that was an important thing to have to keep the expansion cohesive as a whole. Anyhow, originally it worked such that if you got to your Clean-up phase and had at least two copies of it in play, you trashed all of them for one Gold each. I thought this was brilliant until I realized it couldn't be done without a +2 Actions card. So I had to substitute the second Prospector for one of the basic cards. The Duchy was the last incarnation of that line of thought, and I didn't put enough consideration into it. Conversely, its penultimate iteration was probably more interesting and useful. Take a look:
ProspectorTypes: Action
Cost: 3
+2 Coins.
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If this is in play at the start of Clean-up, you may reveal a Gold from your hand. If you do, trash this and gain 2 Gold cards.
Hmm, so do I play the Gold now, or do I keep it in hand in order to gain more Gold? Unlike Duchies, you're liable to pick up some Gold fairly early in the game. Is it worth having a terminal Silver until you match the two together?
I don't have time to post responses regarding all the cards right now, but I'll pull one out of order before I go.
Aqueduct
Types: Victory
Cost: 5
Worth 2 VP.
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When you gain this, reveal the top 5 cards of your deck. Discard all revealed Victory and Curse cards and put the rest back on top in any order.
Yes, I was the one that posted this card in the other thread. I'm just completely blown away that you thought it was good enough to playtest and that it tested so well! Thanks!
I admit that I'd assumed the Aqueduct-or-Duchy choice was integral to the card. I figured that if I priced it at $4 that there would be no strategy there at all; with $4 you'd buy Aqueduct and with $5 you'd buy Duchy. Since you've actually tested the card, though, I'm happy to defer to your experience. At minimum, I'll test it at $4 myself.
I'll post more when I have time. Thanks again for your insight!