I think they should take a level of inspiration from RTS balance, principly "equal but different", while allowing for a loose balance point so that it never has to be perfect.

To give a couple of examples, strictly in principle:-
PLD: THE mitigation tank, all about using that shield to reduce damage taken and has minimal - if any - heals.
WAR: THE heal tank, basically the reverse of the PLD, it has many heals (though maybe not quite as powerful as now) but minimal mits.
DRK: The parry all the things tank, tapping back into it's roots it seeks to maximise it's parry chance and shields what it has to while turning incoming damage into outgoing damage be that with an HP scalar or through it's gauge.
GNB: Then becomes all about evasion, I'll leave it to more creative minds to flesh it out though - I haven't ideas.

In terms of healers I'd offer:-
WHM: Raw power heals. Heals infrequently but for vast amounts when it does and foregoes regens.
SCH: Stays more or less as it is with an added focus on shields since my SGE model doesn't step on that territory. Prioritses planning ahead, possibly expanding on the Excog model so more of SCH's own abilities have extra activation conditions. Whether those are for the ability activation at all or in the vein of AST's Earthly Star doesn't really bother me, leave it to the SCH pros.
AST: I'd say double down on regens, timed effects and most of all card play, people have said on these forums about how it used to have various card modifiers and abilities - Bring them back so that cards can become the central pillar of the job.
SGE: Mits and patches, focuses mainly on reducing damage and applying smaller heals when people do take damage, maybe reverse the current kardia model so that heals do a little aoe damage based on the recipient(s). (It's not the best idea, I'm sure, but I'm not creative so I'm sure someone else will think of better)

Similarly the 2 minute meta, while I wouldn't through it out all together, I would throw out the one minute part so that the jobs can have more peculiar cooldown timers.