Materia sound cool as long as it is not "Evolith" system.

Materia should give some offensif skills (or buffs) on weapon, and defensif skill/buff on armor.

You can even make the whole new system abilities based on materia (and i think, it will be the principal interest of the game after that).

Notorious Monster could drop some Materia stronger than what you can get from craft, or armor/weapon that can socket more Materia.

If you make Materia the essence from gaïa... With this new ressource, we have à good reason that garlemard empire doesn't attack us.