Here is a BLU concept that should be possible to balance with the regular jobs, while also keeping the learning and spell selecting aspect that is key to the job.
https://i.ibb.co/K2mjHzY/Blue-Magic-Spellbook1.jpg
The idea is you pick any 3 elemental spells that flow from one to the other to build your basic combo.
You get minor focus for doing the combo, and that focus can spent on a 2nd combo built on the same principle, flowing from one element to another. The 2nd combo gets a damage bonus to represent your
powering up.
By doing the two combos and moving through the whole elemental wheel you can gain a major focus, those can then be spent to unleash your bigger spells.
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Could be an add on to current BLU. BLU could become a class with the job crystal being a story McGuffin, at some point in the story a job trainer (level 30 maybe?) comes along and gives you a
focused job stone that be used to change the class over to this.