Overall, I feel that the theme of discarding cards (which I've seen a few times now) is okay, but not so overly interesting that it really carries half a set. I mean, each card on its own might still be alright, but if the same mechanic is repeated over and over, it makes the individual seem less interesting in the end. Chancellor for example used to be unique - now that there are Scavanger and Messenger, it's just the worst of three and was justly killed (
*sob*). This is not to discourage doing cards with a specific mechanic in general, just to say that I feel some mechanics are more fit to spread over a lot of cards than others...
That said, I'll try to give some helpful feedback on the individual cards:
Ancient Library (3?)
Action - Reaction
You may discard any number of cards.
Draw until you have 5 cards in hand.
When you discard this card any time other than during Clean-up, you can reveal it. If you do, draw a card.
This technically can lead to an infinite loop where you draw and discard it, but as revealing a Reaction is technically always possible unlimited times per trigger (e.g. putting back and drawing the same two cards with Secret Chamber) it's not actually a problem. The cost seems fine, as while it has a Reaction in addition to always drawing at least one card, it can't be used on large hands as well as Cellar. It's also of course a defense against discard attacks.
Ancient Markets (4)
Action - Reaction
When you discard this card any time other than during Clean-up, you can reveal it to put a Copper from your discard pile into your hand.
Is there a reason this one excludes cleanup? It's not like taking that Copper would do you any good there. But I guess it's more consistent. Considering it has a Reaction and only lacks the buy of Festival, I'm pretty sure it can't cost 4$. After all, it's also more or less a Silver+, which is something that on these forums is often assumed to be unfitting at that price point (although there's no real evidence for why it shouldn't, so whatever - let's go with the Festival reasoning, though).
On another note, I'm not sure where the Reaction even has its uses. It doesn't defend against a Militia, or does it? Technically, I'm not even sure what happens if I have 4 cards in hand and discard this (or Ancient Library, for that matter). Did I discard down to 3 already and can keep the additional card? In that case, it's a pity these don't stack. And even with discard for benefit, which do I use this with? For Oasis I'd rather discard a Copper, which leaves me at the same money and an extra Action to spare. I'd even consider discarding Silver rather than this if I need that Action. So is all it does protect better stuff from your hand if you NEED to discard, but not to a certain limit? That seems really specific.
Ancient Temple (4)
Action - Reaction - Attack
Choose one: Trash up to two cards from your hand; or,
Gain +1 VP.
When you discard this card any time other than during Clean-up, you can reveal it. If you do, each other player gains a Curse.
This one doesn't need the Reaction in my opinion. Not only is the reaction not looking like all that much fun (e.g. if you just Cellar the same one three times a turn through your miniature deck), but it also already seems quite a bit better than Monument given its flexibility (it also can look down on Masquerade, btw).
Ancient Waterways (5)
Action - Reaction
When you discard this card any time other than during Clean-up, you can reveal it to gain a copy of Ancient Waterways.
This seems balanced just fine. I still have a little complaint, though, as this card makes it rather clear where these designs differ from the official on-discard Reaction, Tunnel. You see, cantrips are often considered "invisible" cards, that is, they replace themselves on play. So one could argue you can never have too many cantrips. However, that's not true. As soon as discarding appears, cantrips suffer a loss in their strength. Sure a Pearl Diver doesn't harm my deck - but when a Militia is played, do I rather discard a Silver, or keep the Pearl Diver, hoping to guess whether the card I'll draw for it will be worth it? Tunnel doesn't have this issue -it's a Victory card, and Victory cards have less of a problem with discard attacks already. It also strengthens my deck further when it reacts. With Ancient Waterways, I can't help but worry that it really loses the comparison here. It actually hurts me to discard the card (or have to choose another one to discard instead), and even if I discard it, it doesn't actually solve the problem but arguably makes it worse. Of course this is theoretical, but I hope my point comes across.
Reveal the top 3 cards of your deck. Play an action card you revealed. If you do, trash it and gain a Treasure card costing up to 1 Money more than the trashed card, and put it into your hand.
I admit I find it hard to imagine which strategy or playing style this is designed for. It trashes Actions and adds treasures to my deck, so it helps me build a money deck? But it plays my actions and does nothing if I don't have Actions on top of my deck, so I wants an Action-dense deck? Somehow I feel what this card needs to work and what this card creates are two adverse things, and to tell the truth, I'm not sure the one-shot-super-Herold--Explorer-thing is good enough to make me want to trash my Labs for Gold. At the very least, I find it strange that it wants me to first go for a strategy to enable changing over to another. Sorry.
While this is in play, when you buy a treasure, you can discard this card.
If you do, +1 Buy and put that treasure into play.
Technically, you'd have to play that other Treasure. While there never was a precedent for a Treasure being played mid-buy phase, I suppose this works. It doesn't look imbalanced to me, either. Maybe the wording could be overhauled a bit, but I fail to come up with a better one.
Cursed Idol (5)
Treasure - Reaction
At the end of your turn, if any player revealed a Cursed Idol when you played this card, gain two Curses.
Whenever an other player plays a Cursed Idol, you can reveal this from your hand.
Wording suggestion: (3$) When you play this, each other player may reveal a Cursed Idol from their hand. If anybody does, gain 2 Curses.
I also agree with the others that 2 Curses seems like an awful lot. What I personally don't like about this is the dilemma it puts a player in. If one of my opponents went for Cursed Idols, I an either have him keep those cheaper Golds without downsides, or get one of them myself, which now suddenly has a huge downside. In other words, I can watch an opponent have the advantage, or give myself a disadvantage to destroy his. So my personal suggestion would be to make the bane another card than Cursed Idol itself, preferably somethign available in every game (e.g. Silver, Province etc), and make the penalty less harsh. Something like:
Werewolf, Action, 5$: +4$. Each other player may reveal a Silver, and if anybody does, gain a Copper.
Victory - Action
Reveal the top card of your deck.
If it is a Victory Card, discard it. Otherwise, trash this card. If the revealed card costs...
$2 or less, +2VP.
$3 to $5, +3VP.
$6 or more, +4VP.
It's cool you took care this can't be spammed for unlimited VP, but the Distand Lands remark somebody made is valid.
Earthmound Village (5)
Reveal your hand.
If you revealed no Copper:
Seems a bit weak compared to Bazaar.
Reveal a card from your hand. Gain a victory card costing less than the revealed card and put it on top of your deck.
I'm afraid that even while this costs 6$ itself, you'll usually have to reveal cards costing at most 5$, having to gain Estates. Not good. Then again, you could just as well reveal Copper to avoid the penalty altogether, which still makes for an expensive Lab. I can't quite tell whether it's intended that way and whether that is intended to be a nerf or advantage. If it's an advantage, why isn't it optional?
Gotta leave, will say stuff about the others another time. I'd like to add that while I complained, that's what I always do (including about my own stuff) and that it was high-level complaining for the most part here