Dyes could be collectibles and not part of the inventory. Get them once and never worry about it ever again. Especially applicable for the ones that are purchasable on the cash shop.
Glams could obviously be revamped completely like in other MMOs.
Fireworks should be unlockable emotes instead of consumables.
Materias could need a revamp, we will now have pretty much 8 tiers of each materia that will be irrelevant.
Your available crafting mats could simply be stored in the crafting log.
The currency menu could be revamped and extended to include any kind of tradeable item (the things that you can trade for Materias: currency. The savage books: currency. The extreme trials totem: currency. Even the items that drop from the 2 rare FATEs of each expansion: currency)
The amount of each fish that you have could be stored in the fishing log.
Inventory could be half of the size and basically limited to consumables only.

But most of it is probably never happening.