Here come the results! Entries are sorted by creator name.
Realm by 4estThe closest existing analogue to "Province for $6" is Fairgrounds, I think. Realm is similar in that it rewards variety. Fairgrounds is fun! And this enables a few different techniques. I worry that the gaining restriction might make it a bit too niche for engines - it's not often that you can have an engine turn without having duplicates in play. And it's not really worth the effort since for an extra $2 you can just buy a Province. I think you should tinker with that to increase the viability, though I am unsure what the best way to go is - maybe more uniques than duplicates in play?
Also, since this has a gaining restriction, we need to address rules issues, and that doesn't really happen here. This particular design makes it so that oftentimes, you will be able to gain this on an opponent's turn, which prevents some issues; however you could still have Durations in play. I think a "soft" gaining restriction is a better way to go, like "when you gain this, unless X, return this to the supply", or "if you have any duplicates in play on your turn, this is not in the supply".
Consultant by AJL828The concept here is quite simple, and that's not bad. I think that it can be quite luck-based whether you'll be able to buy this early on; just an Estate in your starting hand may block you. But that's all probably fine; it's not like Consultant is a super high-priority purchase, so you won't be too sad when you don't manage to buy this. It's a solid design.
Beckoner by arowdokThis is a super-Cobbler that requires a lot of setup. It seems quite cool, but it's hard to judge the power level. Since you need to put in some work to make this happen, you won't get it super early, but gainers are at their best early on. On the other hand, the effect might just be strong enough to still make this worthwhile. Probably requires some testing. There are some neat mindgames to be explored here when playing this + Swindler.
The text is a bit too long for my taste, I would cut the Treasure gaining option to get down to 7 lines of text, but that's not a major issue.
Machine/Broken Gear by Augie279I like this design overall, and I like the idea of doing a bad Artifact. It does "you can't gain this" and thus provides some rules problems, and also it feels like you are double-punishing having a lot of Machines in play via the gaining restriction and the Artifact. And it's a bit too much text. I guess what I'm saying is that this design would be better without the gaining restriction, which is of course a bit of a problem for this contest.
Also here's a random rules question: When you play Machine with Broken Gear as Way of the Chameleon, do you draw a card or get a coin?
Capital City by Commodore ChucklesFrom the text, it's unclear to me whether you need to have the two Actions you want to play in hand at the time you play Capital City or you can play one and then draw another and play that. Though since there is no accountability built in, I assume it's the latter. If so, I recommend: "Do this twice: You may play an Action card from you hand twice".
This likes cost reduction, is my immediate thought. I wonder whether this strictly needs to be more difficult to get than King's Court, but then King's Court is pretty good. I still think this could stand having its cost reduced to $6. Overall I like it, Throne Room variants are always good fun.
Weaver by D782802859So here, once you buy the first one, you can circumvent the buying restriction and just get as many as you like. I feel like this could be a bit swingy if you start with Heirlooms/Shelters and one of you can get Weaver on T1/T2. On the other hand, this is a solid card, but it's not going to give a game-deciding advantage. So yeah, this feels quite decent.
Elitist Village by emtzalexThis feels quite restrictive. It has been pointed out that this is about the power level of Port, and I don't think it needs to be so expensive. Could easily cost $3 to make it just a bit more viable. Other than that, I have no complaints.
Financier by FirestixThis is just too weak. Hireling is already a kind of bad $6, and a card at the start of your turn is much better then a choice of $1 or +Buy. It would be a bit better if you were able to delay your decision until the start of your Buy phase, but not much. You might get one of these if you really need the extra Buy, but it feels quite expensive compared to Fair. And since you raraly want more than 1, the buying restriction doesn't do a whole lot.
Agent by infangthiefIn a way, this is like the opposite Grand Market. With GM, once you have the first one it becomes easier to get the next ones, and here it's the other way around. Unfortunately I don't think there is much depth to this card; you just get it when you can. It's not really worth it to miss out on playing a Agent to get more Agents, but if you happen to not have any Agents in play, it's a great deal. So it ends up feeling kind of boring.
Field by jakavAnother Province alternative! I don't think this is strong enough to matter a lot, unfortunately. There are many engines where you don't want any Coppers or Silvers in your deck anyways, and there sure you'll get some Fields (though would you get 3 Fields instead of 2 Provinces? I am not convinced). But otherwise, this isn't good enough to justify sacrificing buying power: If you have a Silver in hand, then you could just play that and get a Province instead, which is also worth more.
Developing Village by JWThis is decent, if a bit vanilla. I'm not convinced that pileouts are as big a problem as you make them out to be; you don't want too many of these. Of course it could be more of an issue for 3-piling, but I think that's also an interesting strategic option, so I would try to keep this. The main problem I see is stuff like Remake/Upgrade. These will definitely lead to pileouts and unlike with Poor House, most of the time you will be happy to trade a Copper for a Developing Village.
Deep Mines by mathdudeI kind of like this, but it falls into a similar trap as earlier entries: Is the buying restriction really necessary? Not being able to play Treasures is enough to discourage a Deep Mine opening, and sure, Deep Mines are awesome once you've trashed down but I think it's fine to let them be awesome... you want these in an engine and you still don't want too many of them because they are still stop cards.
Foundry by NoMoreFunThis is a new spin on "having one of these makes getting more hard". The effect is suitably good, it's simple and effective, the buying restriction is easy to follow. It's good. My only worry is that it might be a bit too overpowering if you have a way of gaining $5s.
Apprenticeship/Master Tradesman by nyxfulloftricksSo Apprenticeship feels kind of unfair from the get-go, since there are 5 and thus you won't split them evenly. When they're good, then then person winning the split probably has a very solid advantage, and that will happen early into the game. I like Master Tradesman, though I don't think it really needs to be packaged into a split pile and could just be a standalone card. And the buying restriction feels a bit redundant; if Master Tradesman is uncovered all the Apprenticeships are already out anyways, so likely you'll be able to get it without much of an issue.
Foreign Exchange by pubbyIt has a simple appeal. Save $3 but you have to spend them before doing that again. I'm not surprised this got a bunch of upvotes. One critique that I have is the effect on openings. First of all, this makes is trivial to open with a $6 (unless you have a $5/$2), and $6s are often designed specifically so you cannot get them in the opening. Secondly, if there are any $7s, then you could open with them on a $3/$4 but not on a $4/$3, and that doesn't feel great for stuff like Inheritance or Forge.
Pyramid by silverspawnIt's not completely clear to me whether you could discard 8 Coppers multiple times at the start of a single Buy phase.
Other than that, there are some combos that make this very easy to get. Just think of Apothecary. Could could probably also make a Counting House thing work but that is fine by me, Counting House can use all the buffs it can get. My main problem however is that these running out does not end the game. So when this is viable, you'll go for Pyramids until the pile runs out and then you end up with a deck with 8 Coppers and 3-5 Pyramids and have to play on with all that junk until you somehow manage to end the game. That doesn't sound like a good time to me.
Abutment by spinefluSo this is like "extreme Bridge". You won't be able to open with it (and you probably don't want to either, considering that unlike Bridge, this doesn't act as a terminal Silver). However, once you get a good engine set up, these will provide excellent buying power. I'm not sure how much I like the ability to just trash the pile with 4 Abutments, that feels like a way to lock your opponents out of the game. I think I'd prefer a version where you need to return this to the supply.
It's also kind of mean with some interactions on another player's turn. Messenger + Abutment? Sure but not for you! Swindler hits Abutment? Sorry, I do not have a replacement for you today.
Huntsman by TiminouVery straightforward. I think the restriction is not enough to make a $6 card for $5; with extra buys it's trivial to get around. And here we have another "you can't gain this" without rules clarification, which I explicitly stated you should consider.
Magna Carta by UltimateGeekI agree with comments calling this too expansive; Attacks hit hardest in the early game and that's when you won't be able to get this. In order to satisfy the requirements you already need to be able to deal with any attacks, so at that point, what do you even need this for?
What's more I don't think straight up blocking attacks is a very interesting design space to begin with.
Late Witch by Xen3kIt's to have a Curser that only comes into play later on, though I feel like Coven already does a similar thing. I don't like the "when you would" wording, and especially not referring to niche cases like exiling/trashing from the supply, that just seems unnecessarily confusing. Another gripe I have is that this drives 2 Supply piles down, and can only be played if another one is already empty, so it will always lead to 3-pile endings. That doesn't seem particularly fun to me.
Charlatan/Bootleg by xyz123I think a Silver+ is a bit much for an Heirloom. The Heirloom also tries to enable getting your first Charlatan, but I'm not sure it does so very successfully: early on Charlatan does not really provide much of a benefit, so at the time you trash Bootleg there is probably another card you'd rather gain, so you'll still end up with many games where this is just a dead pile.
I think this card is designed around the whole "you can't buy this" thing and tries hard to make that work - I understand how that happens. But I'm not convinced that "you can't buy this" is actually interesting enough to justify all the effort.
Shortlist: Beckoner, Capital City, Weaver, Foundry, Foreign Exchange
Winner: Foundry by NoMoreFun