I originally posted a lot of card ideas on Reddit. I figured I'd share the four most popular and elegant ones here.
Rabbits - Action Card costing $2*
You may gain a Rabbits from the Rabbits pile.
Put this on a non-empty supply pile that doesn't have a Rabbits on it. (Cards underneath Rabbits cannot be gained while Rabbits is on them.)
If this isn't in the Rabbits supply pile, it costs 0.
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 Ruined Market. You can rid them from your deck, but then they just clog the supply board even more. These invasive vermin are everywhere!
I'm very tempted to add +$1 to it, or maybe "you may gain a Rabbits from the Rabbits pile, if you do, +$1" but I'm not quite sure. It's difficult to balance this card to be annoying to have to buy when it's on a pile but useful enough that you want to buy it in the first place.
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 - Action Duration costing $4
Choose one: +1 buy and +$2 now, or
+1 buy and +$3 at the beginning of your next turn.
A slightly more complicated version of this is a Victory/Duration card.
Young Nobles - Action Duration Victory costing $3
Choose one, +2 actions now,
or +2 cards at the beginning of your next turn.
Each of these options by themselves isn't too amazing, but the option value of this card can helps smooth out rough engines. I'd really love to see other people's option-duration card ideas. It's such an elegant concept but underutilized.
And last, a landmark:
Martyr's Tomb - Landmark
+8VP to the player with the most curses (or tied for the most.)
Here you'll want to win the curse split. So it's tempting to think that you can wait until the last turn and buy a spare curse, netting you +7VP, but of course, your opponent knows you'll do this and buys one in advance. To counter this, you'll have to buy one in advance too. This becomes a curse race. Start buying curses too early and too often and you clog up your deck. Wait too long and you lose the points. Of course, you can also get a bunch of spare +buys and simply threaten to buy curses. Maybe then your opponent will overreact on the curses and you can just ignore the curses entirely and win with a leaner deck.
I am BGG's official honey trap
Moved from General to Variants.
Rabbits sounds like a fun idea.