I would like to see one of two things...
1) when a card is put into a deck put the card into the deck and alow the # of cards in that deck to be changed without affecting the # of those cards in the inventory.
2) a colum could be added to the inventory list for each new deck allowing users to indicate how many of each card is in each deck.
Just a couple of thoughts. I hope they make sense... it's late and I'm not sure it's reading right... 
[mtg][/mtg]