An idea for a new class (potentially limited job) Freelancer/Mime (not mimic)

Inspired from Final Fantasy 5, and as we can see in the Dissidia games wielded by Bartz.
The idea of this class is simple; Sanctioned multiclassing.

Basically you master a class (max its level),or even have level checkpoints for the class, and once you master the class you get abilities/spells/passives (traits) from that class; You can mix and match the traits (2-3 max) to promote your own playstyle, but the spells and abilities can be used freely, and without needing to equip and unequip abilities like blue mage; once you earn them -- all you would need to do is press a button mid combat to switch your stance/weapon.

The Freelancer/Mime is weak starting off, and doesn't really even become that useful until you've mastered a few classes first, but once you do it becomes awesome, allows the player to safely multiclass without disturbing the entirety of the current class system, and encourages players to play and master more than one class.