The gathering of Cards has been done in another game and done well. Game is Ragnarok Online. There the cards are used as our Materia here. Each card has its own specific effect. 1 card gives better attack speed, another card gives your armor immunity to knockback and things like that. Gathering those cards was by killing monsters. Bosses, regular mobs, quest mobs etc. <card name> would drop from the same <name> mob. Just like OP has suggested. I really really hope this is implemented this way as far as the gathering.