Materia should be inter-changeable with any weapon/armor at anytime, so your goal in the game is to collect the best materia for your weapons/armor and find the best mixes of those materia, Although materia shouldn't be an inventory slot, but kept in its own materia items, and duplicates cannot be gotten, or traded. This makes you earn these materia and gather them and build your own weapons/armor.

Each ranked item should have at least 1 materia fitting and who knows the limit based on the future of the game, but maybe 1 materia can be inserted per 10 optimal ranks. This allows for more combination's to get different results. This does open the door for spells, abilities, and many other items just by combination's or materia and/or a materia itself. I do think this would give more strategy to the game and promote collecting of materia and building your own style of play.