each time a card would be destroyed (excluding selling and using consumables), prevent it from being destroyed and gain 0.2x mult (or 0.1x if 0.2 is too strong)
yes another one of the quite common "prevent card from being destroyed" suggestion
(card here can work for both joker and playing card)
Potential downsides to add if it's too strong but changing the xmult gain is too hard to balance:
A) Resets after beating boss blind (A la campfire, but might have to buff the xmult gain slightly since campfire gets 0.25x per sold, and selling is way easier than destroying)
B) The card destroys itself at a certain Xmult threshold like 5x or something
C) Xmult cap preventing it from going above a certain threshold (kinda boring)