I like using decks to show what box/binder a specific card is in, but that clashes with some of the features regarding cards in decks. A way to put them in folders without counting towards "cards in built decks" and allowing for the same tools Inventory does would be cool. Or maybe a way to simply add a "location" tag to cards and filter by it?

I'd also want this very much, it'd be very useful as I sometimes have different format decks sharing cards across them.

Desperately want this feature!

Vote for that feature.
Having something like tags in invetory would be useful in many cases.