Many of these are reasonable. I'll give my reaction to each of them, and forgive me if I'm a bit harsh. That's just how I roll.
Base:
Windmill
Action - $3
+1 Action
You may discard a card.
Draw up to 5 cards
This seems fine, if a little unexciting. It's meant to go in the Base Set, though, so perhaps bland is the order of the day. It steps on Cellar and Library's toes a bit, but it's probably different enough. My main concern is that it doesn't
look that useful outside of games with discard attacks and disappearing Actions. Even if it is, that's not a great thing in the Base Set. Chancellor already causes confusion there.
Overall, I'd prefer a card that was less similar than the existing Base Set cards. (Also, the Base Set needs more $5 cards.) If you keep it, I'd make the discarding mandatory.
Intrigue:
Lady-in-Waiting
Action/Attack- $5
Choose one:
+3 Cards, +1 Buy
or
Gain two silvers, each other player gains a curse.
Intrigue already has two cards that can give out Curses. This is definitely an interesting card, but I'd prefer a different attack effect.
Seaside:
Buoy
Action/Duration - $2
+1 Action
+1 Buy
Reveal the top card of your deck and put it face up on this card. Cards that share its type cost one less next turn, but not less than $0.
Very cool idea. Do you mean 'cards that share a type with it' or 'cards that share all its types'? I prefer the former, personally. It might be more compelling as a terminal $5 card that lowered costs by $2, but that also might be too abusable with Throne Room, etc. Definitely test it as-is.
Alchemy:
Perpetuum Mobile
Action - $2P
+3 Cards
+1 Action
Discard down to 5 cards in hand.
Not a fan. For one thing, it seems to really conflict with several Alchemy cards. Scrying Pool, Apothecary, Apprentice, and Alchemist are all about getting a large hand size. Even for the cards this combos with, it's marginal. I guess you could discard Actions from your hand so Golem could find them? And unless you're discarding only Victory and Curse cards, you're probably losing more than you're gaining with Philosopher's Stone.
Also, any new Potion cost card should really wow me. They're expensive and hard to get. You can do things with Potion-cost cards that you can't get away with on a normal card. See Golem and Possession, for example. This card is just too vanilla by far.
Prosperity:
Dowry
Treasure/Reaction - $5
$2
+1 Buy
When you would gain a card, you may reveal this from your hand. If you do, gain 1 VP instead.
This basically just allows for infinite VP churning engines without ever approaching an end game condition. It's way, way easier to use than Monument since it's a Treasure/Reaction rather than a terminal Action.
Cornucopia:
Illusionist
Action - $4
+1 Action
You may discard any number of differently named cards. For each card you discarded: +1 Card, +1 Action, +$1, or +1 Buy
I have two issues with this card. First, the differently-named bit seems very tacked on (and you'd need to say, "You may
reveal then discard"). Second, do you choose the bonuses one at a time, or all at once? I assume one at a time. It's a little confusing.
Even once the card is understood, I see it creating huge amounts of AP. It's like a mega-Pawn. "OK, I choose +1 Card. Now I need to reevaluate my new hand before choosing another bonus. Hmm…" Once the player has decided to stop drawing cards, the rest is a bit faster. Still, I'd reduce the number of options.
I notice this is the third non-terminal discard card in the set. Was that an intentional theme?
Hinterlands:
Land run
Action/Attack - $5
+$2
+3 Cards
Every player (including you) discards down to 3 cards.
When you buy this, reveal your hand. For every victory card you revealed, gain a silver.
Cute on-buy effect. I always enjoy an opportunistic buy. I don't see why it can't be on-gain, though. You could always add "you may" to avoid mandatory Silver gain if someone were to Ambassador it to you.
What is with the self-discarding, though? A card that makes you discard down to 3 cards is pretty harsh, even with the +$2. It probably seems worse than it is, but that in itself can kill a card. You should playtest it. Maybe it's not as harsh as it looks, but I think most players aren't going to think discarding down to 3 cards is fun.
Dark Ages:
Arsonist
Action/Attack/Looter - $3
+2 Actions
Choose one:
Each other player gains a Ruins
or
Trash this and gain a Bonfire.
Bonfire - Action
Return this to the Bonfire pile and trash any number of cards from your hand. Gain two Spoils.
This is not in the supply.
Even though it's Ruins instead of Curses, a $3 junking attack is harsh. And it's non-terminal! So everybody opens with 2 of these, junks until the Ruins run out, then converts them to Bonfires and tries to trash the Ruins, rebuilding with the Spoils. Cute idea, but more than any other junker, this seems like a mandatory opener. If you choose not to participate, your deck will be instantly ruined and your opponent will get to use all of their trashing on Coppers and Estates. It seems even worse than Sea Hag or Ambassador in this regard.
Guilds:
Landlord
Reaction/Victory - $4+
2VP
When you overpay for a card, you may discard this from your hand. If you do, you may add $2 to the amount you overpaid.
You may overpay for this. If you do, gain another copy of this for each $2 you overpaid.
(setup: there are 6 copies per player in the supply, with a minimum of 12 copies).
A little niche, perhaps? It allows you to completely ruin your deck in the endgame with a few massive VP gains. Maybe that's interesting? I guess it combos in a neat way with other overpay cards, but that's a really specific interaction which only works with 4 other cards in the entire game. I'd prefer a Reaction that got you Coin tokens.
Promos:
Saint
Action - $4
Trash this. If you do, put an action card from your hand on your Saint mat. Whenever you play an action card, add +1 Card, +1 Action to its description if you have at least one copy of it on your Saint mat. Return the cards to your deck at the end of the game.
Neat idea. +1 Card/+1 Action may be a bit much. I'd try +1 Card first and go from there. Instead of adding the effect to the description, just say, "When you play an Action card, +1 Card if a you have a copy of that card on your Saint mat."