I'd like to see more cards that involve choices and more dual-type cards, a la Intrigue. Perhaps a card which is a Curse and something else?
I know it probably won't happen, but a couple more Durations and VP-chip-givers would be nice. (I suspect more VP chips are more likely.)
I'd like to see some trashers and curse-givers which are actually kind of mediocre and only make sense as situational buys.
I'd like to see an attack which lets you look at other players' hands and discard a card from it. With restrictions, of course (at a minimum it should only hit people with more than four cards in hand), and it'd have to be really expensive.
I'd like to see a card that makes buying Bureaucrat worth it.
I think that mostly covers it.
I had an idea like that a while ago, posted it on BGG.
Combining Curses with Actions and Treasures seemed like fun to me, but we have to be careful with the Curse dealers to not make them too powerful.
I will give some examples:
Cursed Loot
Treasure - Curse
Cost: 0+ $2
-2 VP
This gives players a hard choice in the beginning when someone plays an early Witch. They may take it instead of a normal curse. I feel the - VP needs to be between 2 and 3, because with -2 it will almost always be chosen over a Curse and with -3 it might never be chosen. Haven't playtested any of the cards I propose.
Haunted Mansion
Action - Curse
Cost: 3+ 3 Actions
- 2 VP
Could be a very fine chaining card
Black Magician
Attack - Curse
Cost: 4+ $2
- 2 VP
Every other player gains a Curse other than Black Magician.
Actually it is an Action - Attack - Curse, so maybe this one doesn't fly and we can't have Attack - Curse cards since every Attack is an Action.
Blood Contract
Victory - Curse
Cost: 6+ 5 VP
At the end of the game: -1 VP for every 10 Victory cards in your deck, rounded down.
Promotes keeping your deck clean with mostly Provinces / Colonies, counteracts Garden rushing, not really satisfied with this one, but it's hard to find a decent Victory - Curse combo.
I just made all of these up without much thought, they obv need a lot of tweaking, I only posted them to demonstrate the uses of some X - Curse cards.