I like a lot of these. Playing off what others have said, here are my new card type ideas:
- Rule-altering cards -
Each player can have one in play at a time. Playing one costs an action, and if it's replacing one already in play the old card goes back in the discard.
Some example ideas:
* During clean-up, you may place one card you have in play on top of your deck. If you do, only draw 4 cards during your draw phase
* Draw 6 cards during your draw phase. During your buy phase, all cards cost +2 coins.
* All players draw an extra card during their draw phase while this is in play.
* While this is in play, the game does not immediately end when all Provinces are gone (it still counts as an empty pile for ending on piles).
(these are probably not balanced, but you get the idea).
One idea I had is that these are all always available for anyone to buy, but there's only one of each (like Prizes) and they're not in the supply.
- Bad cards -
These are cards that have some negative effect that you are forced to play. The idea is that at the start of your turn, if they're in your hand, you're forced to reveal them and do something that hurts you, but they have a good amount of victory points to offset them (not sure how many VP would be needed to balance). Possibly could also have a different bonus, like be worth a lot of +coin.
Examples:
* All opponents draw a card.
* Return the last card you gained to the supply (or trash it, if it is not in the supply).
* Pass a card you gained last turned to the player to your right, his choice.
Not sure if anyone would buy these. On the other hand, if it's late game and they're worth a lot of VP, the negative effects might not even come up that often before the game ends, so they could be too powerful.