I see a lot of people post requests for their Discipline of War / Magic classes to be more useful at level 50. This is largely because people want to be able to use more cross-class skills than Jobs allow, so they can make a character that feels more unique and special to their playstyle.
So what I think would be cool is if they implemented an Onion Knight Job that is able to use any skill that has been unlocked by a Discipline of War / Magic class (but no Job abilities). The Onion Knight Job would have well balanced stat growth so they can be equally good as a dps, tank or caster.
The Onion Knight would have only 5 abilities that can be toggled on and off but only one can be active at a time:
"Paladin Mimic" - Clone of Shield Oath, except also gains 25% more HP. Appropriate for a tank
"Black Mage Mimic" - Gains a bonus to INT at cost of HP. Appropriate to be a caster.
"Dragoon Mimic" - Gains a bonus to STR at cost of INT. Appropriate for a melee DPS.
"Bard Mimic" - Gains a bonus to DEX at cost of STR. Appropriate for a bow DPS.
"White Mage Mimic" - Gains a bonus to MIND at cost of INT. Appropriate for a healer.
The Job would be kind of like Super Novices in Ragnarok Online; something that is kind of hardcore to play, and meant to add additional challenge.
I'd even propose that when using the Duty Finder as an Onion Knight, the player has the option to enter a special "Onion Mode" where only other Onion Knights are allowed to enter the instance. In this Onion Mode version of the dungeon, the dungeon drops would change to be special Onion Knight type armour and weapons. This would add a new level of challenge to the game for those players who really love the idea of cross-class skills.
I'd make the requirement to unlock the Onion Knight Job with the player needing to obtain at least level 35 in all Disciple of War and Magic classes, and having all the current Job Crystals.