or completing a collection can give a buff to the pawn that completes it. Some small stack-able buff like 1% attack, dodge, etc. In that way people will hunt for the cards. You can balance it by making legendary ones extra rare, in that way completing a collection will be harder while rewarding at the same time.

Maybe make collections their own recreation furniture? bout all ya can do short of making a mini game. be cool to see some old school tetra or triad from the old PS1 final fantasies :)

Idk if it was you or the original modder that was thinking of something else to do with them, but sending in a set of cards, or a complete collection or something (maybe via quest) could give some good rewards. Idk, I just like having neat things :)