Be able to clearly mark a card as still needing to purchase so you can easily see which cards in a decklist you still need to buy as you build a deck.

The Wish List was designed to do just this. Is it not working helping you track this type of thing?
I didn’t know about the wishlist mechanic! That’s helpful but I feel like it could still be more helpful and easier to track which cards you need to buy or already have while you’re looking at the specific deck And not going to the separate wish list page
I didn’t know about the wishlist mechanic! That’s helpful but I feel like it could still be more helpful and easier to track which cards you need to buy or already have while you’re looking at the specific deck And not going to the separate wish list page
I don’t think the wishlist handles ‘Cards I want to buy/bought per set’ sort of problem.
I interpreted OP’s idea as allowing the wishlist to differentiate cards that you need to complete projects, versus cards added for other reasons. I’m imagining the collection icon would change to something like a stack of cards, the same way when viewing a deck it changes to a shopping bag based on whether the card is in your wish list “deck”(but with reversed logic.)
You’d probably want it turned off by default, and have the user manually turn it on for specific decks, packages, or boards, perhaps with an upper limit in place; that way the site doesn’t have to index a huge volume of stuff for no reason.
Right now my wishlist is split between cards I think are neat and want to collect, and cards I’ll need to complete a deck I’m working on; I’ll probably buy those groups separately so automating grouping them using my saved decks would be handy.