I like mob cards. The idea of getting rare/unique drops from killing mobs is a great way to incentivize hunter roles and keep things fresh.

A couple of complaints from the community is that they clog the inventory and will inevitably lose value over time.

Complaint #1 can be addressed with a dedicated mob card inventory (I know that /chest works fine, but hear me out).

In terms of depreciating value, I'd propose a cosmetic set unlocked by turning in mob cards (or sets of mob cards) in return for mob-themed cosmetic items. This not only adds an item sink for mob cards, but also creates demand for cards past a collector's novelty item.

I think that these two proposals can be introduced in conjunction with one another. A card inventory can double as a collection page to view owned cards as well as missing cards in a particular set and can serve to streamline implementation of a collection system.