I think it would be awesome to have the ability to add a "may cast" card to an item. If we could make and then insert a special gem slot for a treasure card to be inserted. You could even have it so the player chooses what kind of actions may activate it, attack, defense, healing... where only one type of action would enable the "may cast" option.
Since we currently have Square, Round, Triangle, Tear, and Star shapes, I could easily see other styles such as Diamond, Hexagon, or Hourglass for instance. These could all be type of Gem slots that can be embedded in your gear through crafting or something. This way if you have an item without a Gem Slot you could add one.