It won't happen because Yoshida said he'd rather add new job than specs.
From a technical point of view, a Job a pretty much a spec.
It took WoW many extansions and struggle to make every job very unique. They only achieved that in the past two expansion and even before, some job litterally had no identity (Disc priest for instance)
And some were just copy of other spec with different twists (Shadow Priest, Affliction Warlock, Destruction, Fire Mage)
If we go back to WoW vanilla and TBC, while every class technically had 3 specs. not only were only 1 viable, but the other were so poorly designed that there was virtually no differences between the spec beside the color of the spells.
Frost and Fire Mage were exactly the same thing for instance, one spam frost bolt, the other spam firebolt.
To come back to FF14, Yoshida rather implement new Job than Specs.
They'd rather implement the Blue Mage than a tanking spec for the Black Mage.
It is a choice like another.
The good point with this design is that it leaves them a lot of room for new jobs every extansion wereas WoW and other game rarely get any new class because a single new class is equivalent to 3 in FF14.
When they introduced the Monk for instance, it was packed with 3 specs, so 3 totally different design with just a few spells here and there as cross spec skills.
I prefer the Job system, at least I know (or hope) that since FF14 is kind poor in gameplay diversity (13 "spec" per say, with would translate to 4 class in WoW), they can add more easily 2-3 jobs every new expac and keep things fresh.
We'll get 2 dps in Stormblood,
Perhaps next one will have 1 heal 1 dps.
then after 1 tank one dps.
Ultimately, this will also help fuel that FF "job" feeling where you have tons of jobs too chooses.