As of the release of patch 6.0, belts will no longer be equippable.
Correct me if I'm wrong, but the given reason for this is that it's just a stat boosting item that has no appearance on your character. The dev team will essentially be removing belts and reallocating the stats to other items (alongside the stat squish).
So, if that reasoning holds true, why do we still have materia?
They fall into the same category as belts in terms of just applying stats, and also have no visible effect(s) on the character. Shouldn't materia also be removed and reworked into something more...exciting?
On one hand, this removes a bit of the customization from gear (which can easily be implemented in other ways). On the other hand, removing/reworking materia would open the way for a much more exciting system.
As an example of what could be done;
When you get a piece of gear you're able to select which secondary stat you want (points would be preallocated). Affixing a piece of materia gives that gear a special bonus. Having enough bonuses or bonus combinations gives your character some collective form of buff.
Ie, affixing only critical materia to all of your gear would give you +10% critical damage (as long as all of the gear is equipped) while melding something like half speed and half critical would give +15% critical hit rate.
This system would be pretty in-depth and complex, but would give materia use outside of being a "stat boost," as well as add varying levels of customization and experimentation.
While this is a pretty extreme example, my point is that I believe materia definitely needs some kind of change if the dev team keeps in mind the same philosophy they're applying with belts. I personally find materia very dull, and often forget about it as a whole until I reach the tentative item level cap. I'd love for it to have more life breathed into it.