I would like to see some sort of job advancement (not merits) mission/quest/battle once we hit level 99. This is an idea that while similar to Final Fantasy I's job upgrades, but would be different.
FFXI's AF1 armor mentions alternate names (or titles) for the jobs we play as, which is more or less a title that's said but not really done anything with.
Black Mage = Wizard
Red Mage = Warlock
etc.
In FFI you were able to advance your job to an upgraded version which granted new spells and abilities. e.g.
Black Wizard got new spells, Warrior became Knight and gained access to low tier White Magic and able to equip all weapons and armor.
So basically, tl;dr:
I am suggesting a new job advancement system for Level 99 jobs that indicate mastery of a job. You would have to complete a challenging (but not impossible) quest in a new dungeon [The Citadel] and fight a final boss in order to earn the right to your new job title.
Once completed, you would gain access to powerful new spells and abilities earned via Trial of the Magians or story-based quests. Subjobs would be disabled (or very gimped) on the "new jobs" in order to maintain game balance.
Black Mage => Black Wizard (WIZ)
White Mage => Devout (DVT)
Red Mage => Warlock (WLK)
etc.
Feel free to add to this.