Jobs are just upgrades to a class.
A job in itself, doesn't do much; you need to unlock it's skills, with quests.
PLD just enchance GLD defensive capabilities. There's no need to make PLD another tank class.
In case they add SAM as branching from LNC, it will just boost some capabilities of LNC. There's no need to add another DPS class, that does the same thing.
I'm guessing SAM and Parivir will be 2 different jobs, from some other class though.
It would only make sense to add new jobs, that start at lv 1, if they learn 16 skills, and traits, of their own, like classes do...but they don't; they just get 5 skills, to specialize the classes.
For ACN we got SMN as DPS, and SCH as healer; in future they may add some support job to ACN instead.

In ARR classes can learn 2 skills, from class quests. 1 from lv 15, other from 30.
LNC can learn Piercing Talon (Ranged attack, by throwing javelin), and Phlebotomize (Dot skill)