[EenKogNeeto]

Spell Cards exist in the lore via spell thiefs, So this isn't too outlandish. I'de say make them allow you to cast any single spell using a battle item. Limited belt slots would keep it from ever being particularly OP, Maybe make them 4m to be careful and to represent the fact you have to use a card instead of just casting the spell normally.

I'de err towards just letting people craft them for existing spells rather than adding new effects and stuff.