My cards aspire to be the sequel to Intrigue. Lots of choices and duality. I call it Dominion: Conspiracy.
Card images by ThetaSigma12 and myself. Click to enhance.
Rabbits
You play this card and it goes on top of another card, preventing your opponent(s) from gaining cards from that pile until it's gone. It doesn't make players gain a curse, it doesn't even make cards cost more, it just makes the first card of a pile essentially cost two buys (or gains.)
But what if there are no +Buys? This could get quite annoying. Luckily Rabbits has +1 Buy, so there's always a card in the supply to alleviate that. Fight fire with fire, I always say. Of course, Rabbits aren't quite fire. When you fight Rabbits with Rabbits, you get more Rabbits. You didn't remove a Rabbits from the supply, you just moved it to a different supply pile. And now they're in your deck too, essentially functioning as a one-use buy. You can rid them from your deck, but then they just clog the supply board even more. These invasive vermin are everywhere!
You can also put this on the Curse pile and cursing attacks no longer work until that Rabbits is gained. Somebody playing a Jack of All Trades or Trader? Put Rabbits on the Silver pile.
Here's another animal card that multiplies:
Wasp (Supply = Same as Curse supply)


Turn extra buys into temporary junk for your opponents.
My next idea is about option-duration cards. The idea is simple. You choose between something tempting now, or something better next turn. Here's a simple example.
Bond
Here's one more:
Inventor
A super powerful card that you don't always want to play. If it's in play, you can't buy that super powerful card again. If you choose the stronger option of +4 cards, that's two turns that you can't buy more Inventors. Its power nerfs itself.
Mulligan
Collection
Shortcut
Guard Tower
Hound
Of course, you don't have to name one of those types. If you're desperate for an attack or a traveller, you can forgo the bonus and simply dig for that specific card.
Loot/Cavern

Pupils/Campus

Locksmith 
Locksmith unlocks your duration bonuses. Get tomorrow's bonuses today! For example, you can play Wharf and get +2 Cards and +1 Buy. Then you play Locksmith to unlock its additional 2 Cards and Buy this turn. (You discard Wharf at the end of this turn.) Turn Caravans into Labs. Turn Caravan Guard into Peddlers. Turn your Merchant Ships into +

. Sometimes you don't want to fast forward your duration cards. For example, you want the attack bonus that Swamp Hag gives, but that's okay, it's nice to have the option. For cards like Champion and Outpost, this won't give you anything, but that's okay too; it doesn't mess anything up either. Sometimes there are no other duration cards in the supply, that's also okay because you can play it on itself.
Jetty
King Midas
Promotion
Accomplice
Ivory Tower
Why would you discard first? When you have a small hand. If you only have 2 cards, it's much better to discard then draw 5 than to draw 5 then discard back down to 2.
Cipher
Scepter
Cooper
These are cumulative, so getting a Copper from a Cooper gives you a Card, Action, Buy, and Coin.
Lost Coins
Trap
Angry Mob
A $2 attacker. Quite vicious if you play 3 of them.
Warlock
Double Cursing. This may be too powerful. We'll see.
Bon Vivant
Bon Vivant. Full of action and everybody wants to be him. In fact, you can be him. Got too many terminals? When you play this, you can turn some of them into Villages!
Jinx TokensWhenever you have 6 or more Jinx tokens, you return 6 and gain a Curse.
Caltrops
Critic
Tailor
Genie (Formerly: Magic Lamp)

I might remove this if it's too powerful.
Kiln
Jungle
The above card (the top half only) is very similar to one of hypercube's events, but we both came up with it independently.
Rearrange
Special thanks to Mail-mi and Commodore Chuckles for helping with this card.
Cabin
A really tempting way to ruin your deck with Coppers.
A "new" mechanic. I use scare quotes because it's probably been done before.
Behold, Payment cards. They do something right now, but you can pay money (once) during your buy phase to do something more.
Cloister
Shield
Installment
Bookstore
Bull
Financier
Camping Grounds
FAQ: Yes, 0 is even.
Removed CardsParrot
This can be completely broken with the new Way of the Chameleon.
Benefactor
Fun card, but empties piles too quickly.
Dagger
Too weak and boring.
Sellout
Donald X copied me again! j/k I liked this card, but Donald's version is slightly more streamlined.
My submission for the +1 Buy contest. Too strong with Goons.
Blockade
I want this card to cost $2, but then it's too powerful in games with attacks. If it costs $3, it's too weak in games without attacks. Interesting double-defense card, though.
Tinker
Apparently trashing a card and putting it back into your hand can lead to infinite trashing with Fortress and Lost Arts/Champion. With Tomb in the set, that's infinite points.
Judge/Courtroom


I invented this before Fleet came out, but now Fleet obsoletes this card.
Magic Coin
Too Powerful.
Settlement
Too similar to one of Asper's cards.
Bells
Was too powerful and way too easy to find Action cards to play. Also it's very similar to a rejected card Donald X. tried already. Oops.
Snakes
Slightly swingy. Not that exciting.
Young Noble
Too weak/boring.
Warriors' Village Action - Attack, 
+1 Card
+2 Actions
Each other player discards the top 2 cards of their deck. If they shuffle their deck, they gain a Curse.
This was a fun card, but had two fundamental problems.
1) Was very luck-based. When testing, sometimes one player would end up with 3 times as many curses as another player.
2) Why Village/Attack? You're going to buy Villages anyway, making them attack doesn't give them any extra incentive.
Cell
This card is probably way too powerful. Comparable to Hireling I think, but cheaper.