Here is my suggestion:
Vary the requirements for each tower.
Examples:
Money Bin: Money generated from Money Bins is higher than X.
Wolf: Deal X damage with a critical Hit.
Herb Witch: Buff a single tower with X Herb Witches.
(Those are only examples, the actual requirements are up to your imagination :-))
You could also make community challenges to decide on the requirements for specific foil cards.
This suggestion might be a lot of work, but you could use them to make "mini"-challenges and give an incentive for The player to play with every tower and vary the carry-tower.
I think something like being able to successfully use that card as your main carry. How it tracks if that tower is your main carry IDK but that is my idea.