One of the main addictive qualities of any good RPG is customization. FF14 attempts to achieve a deliciously customizable game experience through horizontal leveling, the armory system, and player-set attribute points yet comes up short on a wide variety of points.
There are a lot of other RPGs that have a socketing system to allow players to further customize and personalize their gear - Diablo II comes to mind a a very successful game with a solid and entertaining socketing system.
It wouldn't even be "copying" another game's system. There already is precedent for socketing in the Final Fantasy universe as far back as 14 years ago: Materia socketing in Final Fantasy 7.
Now, it can't work exactly like how it did in FF7, but the basic concept of gathering items to use to make your existing gear stronger is a good place to start to add more depth to the current armory system.
It can be as basic as +stats, to perhaps something more exotic like "chance to proc 'stun' on hit", or "leech 1% dmg", or "+2 enmity", or "Sub-class affinity +10" or "add fire dmg", etc etc.
Whether these socketable enchantments are found as mob drops, synthed by crafters, or collected by gatherers, it'll give one more thing for everyone to do, and can help to spur the economy as well.
Items could be socketed by crafters, or perhaps by an NPC for a gil fee (we do need more gil sinks after all...)
Since socketing, in one form or another, is a positive factor in a lot of other successful RPGS, and since there already is a form of socketing in the Final Fantasy universe, I don't see any reason why this idea can't be implemented into FF14 as well.