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.
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.