As I've just recently started the game and to build a collection of cards, it would be nice if on the page of a Deck and during the deckbuilder there was a toggle to overlay the amount of cards owned/needed and darken them if not owned at all. I often like to build decks before owning the cards entirely, and shop afterwards when it's settled on something.
It would help tremendously with shopping for missing cards later.
PD: This is a different feature from the premium feature to Deck suggest based on collection. This would just be a manual overlay toggle when looking at a public/own deck's page.
