In
this thread the idea of negative vanilla boni came up. We already have discard on many cards and I use a card with negative coins. Discard doesn't force you to discard cards which you do not have whereas negative coins can lead to temporarily negative coins (they can increase due to other Action/Treasure cards but after having used all Treasure cards in your Buy phase the minimum amount of Coins is zero).
Negative Actions and Buys should work differently though IMO. You can only play these cards if you actually have the Action or Buy. So for example a negative Action card requires you to play a Village before.
Here are some quick ideas:
??
Types: Action
Cost: $5
-1 Action
+4$
+1 BuySeems like a BM card but as you need a Village it is probably more of an engine card.
??
Types: Action
Cost: $5
-1 Buy
Gain a card costing up to 5$.Obviously this has to be compared to Altar and could be too weak. On the other hand at a price of 4$ it could be too strong and an opening autobuy.
??
Types: Action
Cost: 2P
Transform any number of Actions, Coins and Buys into any number of Actions, Coins and Buys.The wording can be changed. Here the implicit rule is that you cannot get negative Coins, otherwise the card would obviously be totally broken.
I have no idea about the right price. Could be OK for just P. Potion should appear for thematic reasons as this seems like a very Alchemy-ish card and mechanically Potion makes it more difficult to get the card. This is the only cheap Potion card (which I know) that you do not want immediately.
One can play around with this via making it nonterminal or, if the card is too powerful, via only allowing the transformation of one vanilla bonus into another instead of pooling all thre types together and then redistributing them.