Just trying to toy with some new ideas...
Investment
Cost: 4
Treasure
Worth $4
When you gain Investment, put it on your Investment mat.
When a player gains a Province, trash all Investment from the supply. Each players shuffle their Investment cards from their Investment mat to their decks, then remove all Investment mat from the game.
Just like in real life, you invest, wait, and one day it will pay off. I have tried with different cost, but found that $3 or $4 is the best as $5 for a delayed effect is too much a commitment. Not sure if the $4 - $4 ratio is the best though.Assassin
Cost: $6
Action - Attack
Each other players discard an action card, or reveal their hands with no actions. You may play a discarded card among them. If no player discards a card, +1 card, +1 action, +$2
In boards that provides strong attack Assassin is often a good buy, and it is never dead - it is a Grand Market when it isn't working. The best defense is probably spamming your decks with low-cost actions. Concubine
Cost: $5
Action - Reaction
Discard your hand. If you discarded a card this way, +$5
When another player gains a Province, you may trash this card from your hand. If you do, gain a Province, put it on your top of your deck.
This card is centered around Duchy and Province in general - Every plays of Concubine guarantee you a Duchy. I've thought of gaining Duchies as the reaction part, but it seems week, but gaining a Province seems a bit strong, so I have the Province putting it on top of the deck. If it still feels too strong, the bonus can always be Duchy + something, or even Gold. Feel free to comment on these cards!