3 Soul Crystals.
Hear me out.
BLU (dps), BLU (healer), & BLU (tank).
Essentially 3 jobs in one.
Each stemming from the same base-class.
Each has different unlockable Job abilities.
Using Greased Lightning as a Faux Example of how to effectively utilize the same class:
GL (dps) - increases damage/skill-speed by x%.
GL (healer) - increases healing/spell-speed by x%.
GL (tank) - increases dodge/enmity-gain by x%.
Personally, I think PGL is a fitting base-class for BLU though a new one could be created.
Note: though perhaps 2-in-1 with no tank option makes the most sense for BLU... too bad we don't have "True Supports".
