Given that this is, like, a medieval "sword and sorcery"-style fantasy setting, aren't tanks a little anachronistic? :P
Interesting idea. Again, I wonder if it just chooses a random durable card of the appropriate rarity, or if it chooses one from your own deck. If it's the latter, someone could have bank be their only durable card, and have a shortcut to one of the most powerful cards in the game.