It'd be cool if the spirit-bonding changed to give us unique "materia" through rare gear or rare crafted gear. Since the IV/V to an extent is handed to us like candy now, it makes the gear materia system a bit aged for what we are capable of now.

Take the 2.0 Primal weapons, bond those up, extract Garuda's weapons, obtain a "En-wind + Earth Resist" materia, only useable on weapons, grants ability "Adds X elemental dmg and increase resist to y element."

As it stands now, the materia system is a somewhat stagnant system as it just entails "Which secondary?" A system which players can feel like it's worth obtaining other gear because they can use them to enhance other gear would add a bit of customization to the game.

With Diadem Coven weapons, give them a convert ability, only useable on weapons and gives the player a "+25% dmg Diadem creatures" and a "+75% dmg 1/4 of the Emergency bosses." Your utility changes because you might get a bond that goes towards the ice boss.