(This took me over three hours. Phew!)Buried Loot by GendoIkari
$6: Instead gaining a card, discard to gain a card costing exactly $3 more
This is often better than Gold. For example, it converts Curses to Silver and is protected against Noble Brigand. Because there is always the $3 gap between Silver and Gold or Duchy and Province, it is rare to pull the blank. It has some interesting interaction with on-buy effects and overpay cards, but it is too similar to Gold, that it is not worth the kingdom slot.
Aristocrat by segura
$4: Discard to gain a copy of a card costing up to $6 gained by any player
An interesting Smugglers variant (especially in multiplayer games), which may be broken at the end of the game, when Duchies or Gardens are attractive targets. It should say non-Victory card, I think.
Science Fair by hhelibebcnofnena
$5: On-draw lab
Like GendoIkari said, this is difficult to verify. You can put cards into your hand without drawing them (for example with Native Village or Envoy). In fact this would require you to draw each card one by one to avoid a complete chaos. Unlike the normal lab, this cannot be drawn dead. On the other hand, this is slightly weaker with draw-to-X, cannot be Throne Roomed etc. Because it is still too similar to Lab, I cannot let this win.
Economist by Chappy7
$4: Discard to gain a Treasure card when any player gains a card costing $5 or more
This is an interesting one. You first decrease your money density in the hope to get Gold / Platinum later. Simple, but very nice.
Firth Guild by ClouduHieh
$4: Discard to attack to get 2 Coffers and attacker gets 1 Debt. Discard when another player gains a victory card to gain a cheaper card
Unfortunately I have to disqualify this, because of a possible Secret Chamber loop. It is also too political and may lead players to completely avoid buying Attack cards. Please do not be mad at me, I would recommend to read the
Dominion Fan Card Creation Guide to avoid common pitfalls.
Sentinel by artless
$3: Discard to draw a card when another player gains a Victory card. Works as Buy-phase-only Bridge from your discard pile.
It is impossible to keep track of this card. The problem is, that you are not allowed to look through your discard pile unless a card allows you to do so.
Design by Fragasnap
P: When a player plays 1 or less actions, trash for a card costing up to $5P (On buy: +1 Buy)
This is also a nice one, especially in 2 player games, it encourages players to play engine strategies. The Potion cost is ingenious, as it prevents you from buying too many of these and also allows you to gain other Potion cards. Alchemy is an Action-card intensive set, so this fits perfectly.
Blockade by Gubump
$4: Put on top of a pile. When you gain an Action card and use this effect, gain a Gold and a Silver.
This feels similar to an early version of Embargo. A non-terminal one-shot which gains a Gold and a Silver and blocks a pile is not bad, but there were more interesting submissions.
Foundry by mandioca15
$4: Trash to gain two cheaper cards, when another player gains a card.
A non-terminal Feast, which can gain two $5 Action cards, Duchies or Golds is too powerful.
Leasehold by faust
$3: Triples tokens. Comes with +1 VP
An interesting Estate variant. Even if you decide to trash it later, it keeps the Victory Point. Probably too strong with Groundskeeper, Plunder, Bishop and similar cards.
Remote Village by mail-mi
$4: Bazaar, which can only be played after terminals
Remote Village is a Bazaar, but only if you have played a terminal Action before. Unlike Throne Room you actually can play it after the Action card, so it is very useful together with Terminal drawers. However, I'm worried that this could be a dead card in some kingdoms. Overall, not a bad card, but it did not make the cut.
Converter by Aquila
$5: Discard cards for Villagers, convert +Cards to +Coffers
This works well with draw-to-X cards, but Fishing Village does this much better. Interestingly it does not turn Faithful Hounds into Labs. I don’t know where the $5 price tag comes from, but this is normally not worth its money.
Replica by majiponi
$2: Trash to duplicate a card costing up to $6.
A non-terminal Duplicate, which has to be trashed from your hand instead of being discarded from your Tavern mat. Unlike Feast it can only gain a duplicate of another card you just gain. Because it also reduces your buying power, it is hard to get $6 early. It can be very frustrating if your opponent Silver+Replica and draws Silver, 3xCopper, Replica in turn 3.
Lodestone by hypercube
$4: Like Watchtower, but into your hand
Unfortunately, this can empty the Ironworks pile. It also need much support depending on the kingdom. While the idea sounds promising, this just does not seem to work in practise.
Illusionist by 4est
$2: Convert any Action card into simple Library
This works well with Ruins and also see Synergy with Vineyards and Tunnel. Interestingly. Starting Chapel + Illusionist is not as bad, because the Illusionist can convert the Chapel in something more useful. it also helps you to get engine parts, but only if you have enough +Actions, because the big drawback is that this is terminal. Usually Jack-of-all-Trades or Smithy are the better terminal drawers.
Copy by naitchman
$5: Copies a card another player has in play
Unlike Band of Misfits this is unterminal, cannot be drawn dead and not limited to cards costing up to $4. The big disadvantage is, that the card is played at the start of your next turn, which makes it less flexible and does not work well with cards like Library, for example. In the most cases, you want to copy Gold or sometimes an interesting Action card. Definitively one of the better cards in the contest.
Convert by anordinaryman
$2: On-gain: You may trash your hand. On-trash: Gain an Action card without +$ costing up to $4.
This card even does not require the Reaction type. It can be in fact type-less. It is very hard to judge this card. So, it would also fit into the last contest as a Chapel replacement. There are some similarities with Trade or Donate, but this one is more flexible. I can imagine starting Convert/Village_Variant and later buying another Convert to convert the first Convert to an Engine part. The $2 is too cheap in my opinion, because a lucky 5/2 player can gain a $5 card and get rid of three Estates. It would be also interesting to see if an asymmetric game with a player who goes for Convert and another player who does not, works at all. (Asymmetric Chapel games are possible, but hard to win as the non-chapel player.)
Fruit Basket by Gazbag
$4: On-buy Horn of Plenty which can trash itself for another Horn of Plenty effect. Can only gain Action cards
This is harder to pull of than Engineer, but can also reward you with $5 or higher cards. This is usually a late buy to get the missing parts of your engine and requires some preparation and unlike Horn of Plenty or Artificer this one gains only two cards. This card has an astonishing strategic depth, but the more simple cards are also promising. There can be only one winner.
Frontier by Kudasai
$4: Trash to gain a card costing $2 more, card when any player gains a Victory card
Fool’s Gold can this better (and this is only Fool’s Gold's secondary effect). Even Feast is more usable as you can play it much earlier, if you want a guaranteed $5 card.
Retriever by scolapasta
$0: Discard substitute
I was actually looking forward to seeing cards which can be useful in most kingdoms. This one is often pretty useless. While this is a novel idea, other cards were more interesting.
Secret Cove by Frolouch
$4: Trash to gain a Gold onto your deck, when another player plays an Attack card; or discard to topdeck a gained card
The first effect is okay, but either discourages players from buying attacks at all or is even contra-productive when your opponent plays a Thief variant. Another dead card instead of a Silver in turn 3 or 4 is pretty bad and will normally cost you the game. The second effect is much weaker than Watchtower. You don't want this card for $4 in your deck.
Tithe by wittyhowlard
$1: Discard when you buy an Action card to trash X cards from your hand (X is the cost of the Action card). On buy: +1 Buy and this goes into your hand
Overpaying an Action by 1 to exchange your Estates or Coppers with a dead card is interesting. However, you could abuse the on-buy clause and empty the pile to end the game earlier. That is not healthy for the game.
Royal Acquisition by Commodore Chuckles
$5: Trash from your hand to double a fixed-VP Victory card
As name and effects intends, this seems to be a card in the Throne Room family. But in fact this is Distant Lands, but with some subtle differences. For instance, there is no interaction with cards which like Victory or Action cards and it is harder to gain a Province with only four usable cards in your hand. Distand Lands gives fix 4 VP. Royal Acquisition can provide you 6 or even 10 VP, but it is often only another Duchy in case of emergency. Distant Lands is less swingy and already fulfils that role pretty well and I don’t see a reason for having another „conditional Victory card“, particularly as 10 VP are perhaps a little too much.
TL;DRWinner:
Economist by Chappy7
Runner-up:
Design by Fragasnap