Actually, I can think of any number of ways to have more crystals. We could have the return of the elemental crystals, we could have some crystals that exist in the world of Ivalice, but outside the domain of Ivalice the country. We could also bring back the principle of the crystal fragments that were used in FFV, and have those fragments still contain some of their power from when they were whole, and they are how you are able to use judges outside the domain of Ivalice itself: You are carrying a fragment of a crystal with you.