If I could pick any class/job from the entire Final Fantasy franchise make it into this game, I'd probably pick this:

Freelancer (Advanced Job, Lv15 All Classes or something): Gains stat boosts based on other mastered jobs (Meaning completed their final Job quest), but never exceeds the best stats of any other Job. Allows usage of all possible cross-class skills along with adopting the skills, armor restrictions, and the role of the class whose weapon is equipped. Instead of having its own Job skills, it doubles the Cross-Class skill slots, effectively making it the single best option for anyone who values skill versatility over increased stats and skill synergy through specialization.