Going to start off by saying: I'm not a game designer (or amateur at best) and this is mostly just musing about the healer jobs, what I see as problems, and what some possible angles could be to improve it or at least lean into a unique angle within the role.
I feel the healer role in general has been pretty unfocused since shadowbringer's Grand En-Dumbening and maintained that since, and looks to only mildly be changing for some from what's been shown in the media tours etc, especially in regards to downtime activities (since currently a skilled/efficient healer or a healer with a party that doesnt stand in orange puddles is punished with boredom, rather than rewarded)
Running down the classes we have:
White Mage
My take - I lost all motivation to look at trying the job once they replaced the entire job with holy effects, and my priorities were elsewhere before, so it seems i've missed the boat entirely. I know they're big and strong healers, with such an infamously soulcrushing lack of downtime that devteam decided to take everyone else's away than figure one out for WHM.
Downtime activity - none to speak of, as far as i can see? you're generally using a big heal OGCD if needed or using glare for most of the fight
How I would consider changing it - the media tour's addition of glare 4 as a proc ability seems a step in one possible direction: perhaps add another avenue or two to glare 4 procs, and perhaps a secondary angle like PCT's landscape via placing down beneficial zone effects--if y'all arent going to make a proper geomancer, may as well let WHM have some fun with it, and it could bring back some water/earth/wind conjury vibes back until the devs get bored and make it holy again.
Scholar
My take - my dearest love in StB, my precious child maimed beyond recognition. it's previous design was centered around strong and accessible healing OGCDs, allowing it to have lots of spare GCDs to use--and look at that, DoTs are a GCD, and you've got quite a few there to slot in when healing isnt needed (thanks again to those juicy OGCDs)
Downtime Activity - same as the above, but without any GCDs to use other than broil for the most part.
How I would consider changing it - if the devs are 100% set in stone forever that only SGE gets to deal damage while healing, perhaps some attention could be paid to aspects like the completely neglected fairy guage: if using aetherflow or dissipation while there are spare stacks of aetherflow, they are converted into fairy guage, and selene's lost actions (perhaps with some modern re-tooling for general use) are brought back as fairy guage-spending actions instead of simply being lost to time forever.
on the subject of dissipation: perhaps you could gain or still access certain fairy actions (such as those just mentioned, or a few from eos as well) while under dissipation, centered on you instead. this would help address the anti-synergy issue of dissipation getting rid of the fairy to grant a resource used by the fairy while granting a healing bonus while simultaneously reducing what buttons you can use to heal, while foreshadowing seraphism earlier in the SCH's career.
you could consider having some actions apply various debuffs to the enemy, such as minor stat penalties like a weaker feint or addle that are overridden by their larger cousins if used, reducing enemy crit chance as a foil to chain strategem, or other auxilliary means of damage prevention (there's a ton to pick from at various levels of efficacy by sniping minor things from blue mage's roster for example, if damage over time simply isnt allowed)
even with this said, i'd still consider at least drastically changing the new DoT to not be locked behind the 120s cooldown chain strategem, especially when it confuses the required skill's useage (it's intended for important single targets/bosses, which wasting it to allow AOE damage--potentially just before an important single target/boss--is more than a little counter-intuitive)
Astrologian
My take - viewed through the lens of my ex-AST main friend, who has since been pushed out of the game over the course of AST's three lobotomies of [card diversity/card effect alteration], [any and all references to time magic aside from lightspeed], and [literally half of the class' kit] with the death of nocturnal sect (this also amounts to a near-total decimation of all previously-established lore for the class as i hear, what with all the cards now being lies, and there is no 'choosing one's destiny' from the sects and mild time stuff).
Downtime activity - rather little as of EW (you use your cards on "correct" party members to collect lucky charms and skip raid mechanics), but potentially looking up from the media tour changes to card diversity (it's still a far cry from it's initial glory, but any amount of recovery is to be celebrated)
How I would consider changing it - lean further into cards and their use: having a simpler DPS kit can be acceptable if that time is instead spent on arranging your cards to be most effective across the party or according to the current situation. for example, adding a "lord" or "lady" equivalent for the new 'defensive' buff archetype in the media tour, or re-introducing some manner of original AST's effect modifications for effectiveness, duration, aoe, etc.
The initial misunderstanding of the "healer leylines" skill during one of ye olde class showcases does raise the option of actually pursuing that route in earnest, since as I recall that was rather well-recieved at the time.
Sage
My take - it's almost a skill-for-skill copy of previous SCH, but instead of DoTs you get several flavors of identical potency (330 single at time of posting, with everything aoe being ~170 potency after multitarget falloff etc) GCDs for the illusion of choice, with the only exception being phlegma--even the capstone laser is the same potency as everything else!
Downtime activity - using it's various identical buttons as they light up, and mostly just let toxikon do nothing, since you're already slidecasting dosis during movement, and it's worse than dyskrasia for aoe, and they're all fighting for GCD usage
How I would consider changing it - i'd consider moving toxikon to an OGCD for starters, and then take a bit more time to differentiate the various attacks a bit more: moving potency up or down to provide a proper action priority, adding secondary effects (to the kardion target?) for using them, such as a minor regen on dyskrasia, or a small shield on toxikon (you get stacks when a shield breaks, so using it as a "patch" or secondary ablative layer would be thematic)