I thought of this tonight. It's probably been done but I don't if it's been done by Final Fantasy. The concept is acquiring job classes automatically rather than through manual switching and rather than being previously determined.
Here's how this works:
Any character destined to be in the protagonist's group are "essentially" blank slates. Purhaps they have a few developmental "points" toward a specific class that suggests they'd be better suited for it but what they become is up to you.
As the leader, you're free to give them any command one would normally find on a ATB menu: Attack, Magic, Steal, Run Away, and even some others that may not exist like Sneak, Barter, Gamble, etc.
The more a character follows these orders, the more points they get toward a skill.
Let's say you want to build a character to a white mage. That character must use healing and other Holy-elemental spells. You can alter a character's class at any time. Even if they have become proficient enough to cast a Revive-All as a White Wizard, you can have them cast black magic until they are proficient enough to use level 3 spells as a Black Wizard. This isn't to say you're creating a Paragon. Bridges only go in one direction so crossing back deducts points from another class. Using magic, whether Holy or not, increases magic points while deducting physical attack points and vice versa. Reaching a balance is the only way to create a Red Mage. Balancing physical attacks and White Magic will create a Paladin. Balancing physical attacks and defensive tactics will create a Sentinel, etc.
Using skills like Steal and Sneak will increase points toward Thief and Ninja classes. Barter goes toward a merchant-style class, etc.