These are a few ideas that I would like to see happen and a bit surprised they have not already...

FF character outfits and weapons as prizes for Gold Saucer
- There's already a ton of wanting for various FF outfits and GS fits perfectly for such.
- Add a couple every patch or so to keep the prizes fresh, and would keep GS a bit more popular.

Furniture to store gear and possibly other items
- Mannequins to add a single full gear set.
- Wardrobes for gear, Drawers for accessories, and Racks for weapons, 5 or so spaces each.
- Space for favorite glamours, while not really harming the need for retainers.

Furniture transfer when moving houses
- In need of some kind of transfer system when moving into a new house so your stuff doesn't get destroyed.
- People put a lot of work into their houses so losing a majority of it makes it hard for people to move wards/districts or upgrade to something larger.