Well I think with that, we need to discuss what strengths, abilities and combos that can be used. Essentially we need to discuss what the card will cost to cast, what the card's defense is, attack, effects/abilities and so on. We also need to discuss what and how the life points of the player will work. Will we do more of a 20 life point game like MTG, or 8000 life points like Yu-Gi-Oh. That will also determine how much attack and defense the creatures/units will have as well as any other cards that can deal damage. That might help us figure how much a card is worth, or how much mana a card will cost to play it.