For any Dev looking in, here's some Card Customization limits us Forumites have come up with so far:
1) All decks must have a minimum of 5 Attack cards
2) You can't have more than 3 of the same Special card (like Poison, Mark of Death, Shatter, etc.)
3) Some cards should be Legendary exclusive or limited to specific characters, like Retribution, Greater Heal, and Shadowfire, and even then they can't have more than 2 in a deck
4) A Character's rank should have some effect on which cards they can add to their deck; We don't want a level 1 Blood Void to be able to use Void Reflection
And finally, one of my suggestions:
There should be a system of costs for adding cards. For example, in Kingdom Hearts Chain of Memories, you had a limited number of Card Points (which increase as you leveled up). Each card (attack, shield, or special) had an assigned CP cost. That way, the number of CP you had limits both how many cards you can add to your deck, and which cards you do choose to add. This forced players to choose their deck style - do they sacrifice their strong attacks for more spells, or focus on defence?
If there is a system in place already, ignore me!
Since currently, the only draw for catching super rare characters like Cosoma is their excellent default decks, we are worried that Card Customization could break the game. Since Stats isn't out yet, these decks are the only things that separate one Shadow character from another Shadow character, etc.