Please note that I am a bad card designer and a worse dominion player. Judging will be based off of aesthetics and my arbitrary whims rather than general balance. One thing I noticed is that a lot of cards had "Trash a card from you hand / When a card is trashed..." or "Discard a card from your hand / When this is discarded" which does make both halves synergize, but in a rather clunky and overly blunt way. The only card in base to be able to trigger itself is Sleigh (don't quote me on this).
http://forum.dominionstrategy.com/index.php?topic=21668.msg901157#msg901157Thrift Store by Augie
Interesting take on a reaction-farmland. Suffers from "trash a card / when this is trashed" and so it seems that the strategy with this is to exclusively buy thrift stores and then golds. Because it is a treasure, you can gain multiple provinces each turn as long as you have the thrift stores to support it, however this is balanced by the fact that you need a thrift store, a trasher, and a 6-cost in hand. A bit weak if anything.
http://forum.dominionstrategy.com/index.php?topic=21668.msg901159#msg901159Shifting Sands by Weretheruler
I don't really know what to say about this. There's no real precedent for messing with other people's decks other than with spy-style or rabble-style attacks. The whole point is to have VP that moves to the mat, similar to Island or Distant Lands. Island and Distant Lands both are actions so you can play them as soon as they turn up. However, this one is a reaction so you have to have this in hand AND wait for someone to shuffle which might not ever line up in a game. So you are stuck with a double estate in your deck until you get really lucky and then execute a very strange attack?
http://forum.dominionstrategy.com/index.php?topic=21668.msg901167#msg901167Ditch by Silverspawn
I don't get the reference. Moving on
http://forum.dominionstrategy.com/index.php?topic=21668.msg901174#msg901174Thresher by czzzzz
Suffers from "Discard a card" / "when discarded". +3 cards, discard 2 cards is a terrible deal, meaning you are incentivized to use it to discard other Threshers unless one of those Hinterlands reactions is in the kingdom. On the other side, the reaction is an extremely powerful effect: a trash and an Action Phase scepter. I would try to balance the top and bottom a bit closer to each other. It seems that this card will be super swingy depending on if there is a discarder in the kingdom like warehouse, forum, oasis, etc.
http://forum.dominionstrategy.com/index.php?topic=21668.msg901189#msg901189Gorgon by 4est
Suffers from "Reveal a card" / "When revealed" but I am going to give this one a pass because revealing fits the Gorgon theme so well and is not the most common effect. If you want to give a curse upon revealing, you will need a top half or split pile to reveal it. I would suggest split pile because of my aversion to "Do X" / "When Xed". It still is a very flavorful card that might be a bit too swingy if anything. I like it anyways.
http://forum.dominionstrategy.com/index.php?topic=21668.msg901190#msg901190Attendant by LibraryAdventurer
This card isn't good. If you have one of them, you are getting a coffer and an effect. Two of them in hand, you are getting a coffer, a trashing, a coffer, and a horse; or 2 coffers and two effects; for an average per-card of a coffer and an effect (+1 bonus action). Seems like too low of a bonus for lining them up if anything. Maybe i'm missing something, idk
http://forum.dominionstrategy.com/index.php?topic=21668.msg901191#msg901191Loggers by AJL828
I actually kind of like the effect on this one if you ignore the "trash a card" / "when a card is trashed" thing. Incentivizes you to have a variety of cards in play. If anything, I would somehow change this to allow treasures to be copied too.
http://forum.dominionstrategy.com/index.php?topic=21668.msg901194#msg901194Barter by Udzu
I really like this one. Making a pure reaction takes balls and this type of card would make for a great promo. The reveal is good for turning curses into copper, or peddlers into provinces. The discard can return a curse, but is otherwise useless: discarding a card after you buy a card is the same as playing a copper and just outright buying the more expensive card. The exile and trashing are great for province strategies. Overall, an amazing card.
http://forum.dominionstrategy.com/index.php?topic=21668.msg901195#msg901195Spymaster by faust
Reaction-Night gets bonus points too for being a new combo. It's pretty good just on its own as a kind of night trasher: a combo of ghost town and monastery. However, I like the reaction part. It only works if both players buy spymasters or a liason, coffer, or villager card is on the board. Nice of the reaction to replace itself too. However, if you are holding all of your cards, you can react with this forever. Holding all of your cards is a rare enough occurence however that I am letting this one slide as a funny combo. Also has funny interaction with possession.
http://forum.dominionstrategy.com/index.php?topic=21668.msg901200#msg901200Wildebeest by emtzalex
"Discard a card" / "When discarded". Also, for a 3-cost, this thing can draw your entire deck just by itself. I would price it up and then buff the draw.
http://forum.dominionstrategy.com/index.php?topic=21668.msg901201#msg901201Sponsor by Builder_Roberts
Another Patron variant, with the top part comparable to Gorgon. Tiny nitpick is that I don't like it when cards offer net +Cards and also +coins at the same time, which would happen here if you have two of these. Also, what happens if this is revealed with, for example, Bandit? Now your opponent draws a card, that's fine and all well just take the card I'm..... wait. I don't even know what the rules here would say. Do you draw the top of your deck that you are currently holding? Or the top unrevealed card?
http://forum.dominionstrategy.com/index.php?topic=21668.msg901205#msg901205Bunker by NoMoreFun
A Necropolis-Cellar that negates handsize attacks.... interesting. The reaction is useless against most attacks however (witches, messing with the deck, duration attacks, etc). The top part is a bit boring if anything and doesn't really match the bottom. I don't have much to say about this card...
http://forum.dominionstrategy.com/index.php?topic=21668.msg901210#msg901210Gold Lust by BryGuy
Simple enough. A smithy that can be a lab or +buy if you have someting to discard. The bottom is a pirate variant. There isn't much cohesion between the halves since you probably won't be discarding golds to this. Also, I don't see the need to specify non-copper treasure. Seems like you are just removing interactions for no good reason unless I am missing something here
http://forum.dominionstrategy.com/index.php?topic=21668.msg901217#msg901217Bribery by X-tra
Top and bottom aren't very related but the creative reaction more than makes up for it. Seems a bit weak since you need it to be in your hand to get the buy, and the top half isnt anything special
http://forum.dominionstrategy.com/index.php?topic=21668.msg901220#msg901220Cove by xyz123
Only one person will be gaining these and they will be very sad that the other player will not engage in the coves. A funny card but seems like a bit of a trap: if one player has coves, the opponent won't buy them because they get free gains. However, once people are buying provinces, a dead card that gains golds and 5-costs doesn't seem that good.
http://forum.dominionstrategy.com/index.php?topic=21668.25Frontier by SignError
An interesting card which makes the 7 cost work very well. Interacts in interesting ways with provinces and with itself. You wouldn't mind picking up one of these if you spike 7$ in the first few turns, and this also makes duchy dancing very interesting. I like it
http://forum.dominionstrategy.com/index.php?topic=21668.msg901222#msg901222Trade Circle by Aquila
Seems worded in a bit of a strange way: why not say "when you discard a card or put a card on top of your deck"? The reaction is good but can't it loop with 3 of them? A reacts to something else and is discarded. B reacts to A and is discarded, A is put into your hand. C reacts to B and is discarded, B is put back into your hand. A reacts to C and is discarded, etc.etc. Unless that's not how things work... overall the reaction is a bit confusing.
http://forum.dominionstrategy.com/index.php?topic=21668.msg901226#msg901226Open Plain by Xen3k
A very powerful tunnel that costs 1 more but gives you 2 horses. Not much to say about this that hasn't already been said: the issue with tunnel is that gold is kinda sucky, but horses don't hurt your deck like gold can. You will take as many horses as you can deck.
http://forum.dominionstrategy.com/index.php?topic=21668.msg901323#msg901323Harbor by JW
A Trail clone that in my opinion misunderstands the beauty of Trail. Trail is not a cool card because it can react to all these different things, and is so versatile, etc. Trail is a good card because top the says +1/+1. A pure cantrip made good, that turns gainers and trashers-nonterminal. I guess this one does too, but in a slightly messier way.
http://forum.dominionstrategy.com/index.php?topic=21668.msg901357#msg901357Fence by LTaco, a late submission but I am merciful
This one is really interesting. A cantrip(ish) that shuffles itself in for a copper, but you don't mind because its net +1 card +1 action.
Runners up in no particular order: Fence, Gorgon, Barter, Frontier by god knows who I'm not scrolling up again.
Winner: Spymaster, by Faust.
Congrats!