I was going to add this yesterday, but I hit my post cap for the day apparently.

Here are ideas of adding horizontal progression without making 1 set that is 'optimal' or required:
1. Don't make the added fun stats effect damage/mitigation or healing abilities i.e.
a) Increased movement speed (not usable in PvP)
b) Summons a clone, this can either do damage and reduce your damage (for same output), take damage but reduce your health (for same total), or just be cosmetic
c) Frost shield, doesn't do damage or reduce damage, but will apply heavy to attackers
d) Translations/Lore based, equipping item translates some Eorzean text in-game to your native language for added lore
e) High jump, lets you jump higher
f) Increased durability, need to repair less often
etc..

2. Don't add armory chest bloat and multiple sets, but simply have an attunement/infusement/enchantment type process