I don't post much on the forums so I don't know if this has been suggested already (sorry if it has).
I would like to see an increase in solo content geared towards a strategic player. What I mean by this is simple. Far too many people power lvl through their jobs and never really learn how to play their job as a master. You see people walking around with Ifrit or Moogle weapons and you immediately think "Wow, they must be a good player," when in reality they could just have been lucky. I would like to see the addition of a primal or primal-like battle that requires you to solo. It could be job specific but really should test the player's ability to think, fight, and win through the use of strategy. The reward for this type of fight could be a belt or accessory for all I care, but just a little something that says "You survived a challenge all by yourself. Congrats."
Two ideas:
CNJ: A battle where the idea is to survive for a certain duration while having to evade attacks, heal yourself/NPCs, live through a hellfire-like attack, defeat enemies using spontaneous terrain features (Ifrit's erruption-esk, except it does damage to both you and enemies). The reward is a U/U Healing Mag Pot ring.
THM: A battle where the enemy switched its mag resistance (or certain magic could heal the mob) constantly and you have to master your elements/timing/combos in order to win. The reward is a U/U Atk Mag Pot ring.
The idea is to test our abilities once we reach max level and compensate us with something that will show off our talents. This will give people something to strive for within their own character development that others cannot help them with. Of course this wouldn't be necessary for game progression but a very nice side challenge. By all means make it so hard that it is nearly impossible. It could easily be made repeatable but offers a not so awesome item the next time around.
Currently, the game has a lot of "down-time" towards the end of one patch and into the next. Something like this could really soak up an individual's time and keep them interested when server populations are low.