The issue is that as the content ages, healing becomes less demanding, thus the ability to DPS "for free" goes from "if I can" to "well nothing else I can do, so why not"
If there are two healers and two tanks in an instance, then the off-tank tends to switch to DPS unless the content is demanding enough to swap, but with a healer, the entire purpose of the second healer tends to be "revive the first healer if they 1hit-ko'd, else DPS" not "take over when the first healer runs out of MP" . We are still seeing situations in older content where both healers decide they should be trying to help DPS, and letting the tank die in the process. Likewise, situations where you get either two AST's, or one AST and one SCH and both of them keep overwriting each others shields, they're not paying attention.
Any time you encounter content where the boss essentially does nothing for 15 seconds, the healer has nothing to do, since casting any healing during this does nothing. There needs to be a reverse situation where only the healer paying attention can save the party, or the party wipes 100% of the time. There are some mechanics like this already, (where everyone has to stack) but the stakes need to be higher where you have to actually reflect the incoming damage, not absorb it.
If SE wants to add another type of character to the game, the should consider adding one that implicitly acts like a healer/tank as opposed to the red mage of dps/healer-capable. Basically a shield/deflection setup, where what kind of damage they can reflect or absorb is dependent on the shield mode (eg physical, magical) as a skill or barrier cast. eg "reflect physical attacks, absorb magical attacks" , "absorb physical attacks, reflect magical attacks", or "absorb physical and magical attacks", where in the DPS is a consequence of the shield mode's reflection.
eg:
Physical+Magical barrier, Absorbs 5% of incoming magical and physical damage, does not return it to target
Physical barrier, Absorbs 10% of incoming physical damage, and returns it to target (as a number, not a %), caps at players maximum HP
Magical barrier, Absorbs 10% of incoming magical damage, and returns it to target (as a number, not a %), caps at players maximum HP
Reflect only, Absorbs 0% of incoming magical and physical damage, reflects 15% (as a number, not a %) back to target, caps at players maximum HP
The barrier would follow the healer character (like a movable "Collective Unconscious" ), and lasts 18 secs or until any other non-shield action is cast, and then needs to be recast. Thus the healer has to be close to the action. Cancels effects of any other HoT shields cast by other players while players are inside the shield and takes precedence over other shields except Paladin's Passage of Arms .
Then you have "+ convert incoming damage to MP", "+convert incoming damage to HP", and "+convert incoming damage to TP" as a single overwriteable DoT which gives an effect closer to Assize on every tick.
Anyway, that's just a back-of-the-envelope idea. I'll be really disappointed if they add another tank or healer, and it's effectively just a re-skinned skillset of another job again. Because for the most part, AST started out with it's own unique bits, but as of SB, it's basically plays almost exactly the same as WHM until level 50 when you can then switch to Nocturnal sect. The cards are RNG, so you can't depend on them, they're just a bonus if you can manage to use them.