Onion Knight (ONK) Limited Job Proposal

[Seraphor]

OP, the problem with your Onion Knight proposal is that because it gets powers from the other jobs you've levelled you don't play as ONK to progress as ONK.
Hell, even the average player will have at least a good third to half of its kit unlocked when they pick it up.

That's the entire point of it. As a Limited Job, it has a non-conventional growth system.
In this case it outsources it's growth to other jobs. ONK content IS all of the other jobs, just as Onion knight has always been. Even the skills themselves are already animated, the work is done already, it's a non-intensive development.
It incentivises people to play and level other jobs, instead of just sticking to one throughout the entire game. Building upon the games ability to multi-class one character.
It shifts attention from BLU's hunting down skills, to actual gameplay content with the Limited Jobs, invoking the need for Limited job gameplay content outside of hunting down Blue Magic spells. By sharing this content with BLU, both gain from it. A more involved Masked Carnivale, a Limited Job Roulette, perhaps even Limited Job Deep Dungeon access, etc.
It places more focus on combining skills from different jobs. Such as Bane being able to spread BRD's Windbite, or combining Berserk with DRG's Life Surge or Blood of the Dragon, using SAM's Meikyo Suishi to spam Souleater, rotating through Fists of Fire for physical damage boost and Enochian for a magical damage boost when needed, utilising Dualcast from RDM's Acceleration to fire off intsant Flares from BLM, etc.