It would be awesome to have "monster essences" drop from all mobs with a random chance, Boss mobs included at a rarer wait. They could be HQ as well.

The monster essence would have to be crafted with a picture of the monster (you take by targeting it and using a skill), the "monster essence," 1 materia, and a philo item. The crafter could HQ the card, and the stats produced would be based on the quality of the monster essence, the base stats of the monster, the quality of the materia, and if the craft was HQed or not.

Could be a huge boost to the validity of all crafting classes.

I'd also like to see a card spirit-bonding system where the cards can be converted into materia or gil or special potions or vanity items.