I'm all for horizontal progression (it's the only true "system". Vertical progression is just glamour with numbers) but, to work in FFXIV, it would ask more than just adding new bonus types to equipments.

The jobs "system" (again, it's not a "system" in its current form. It's just glorified fan service) would need a lot of rework. A good equipment system is complementary to a good progression system.

Then, the battles design would need to change too. Horizontal progression allows players to come up with their own solution to win battles. FFXIV doesnt work like this : to win, we only need to guess the developpers' shenanigans.