PLD - A Tank with a lot of party healing and mitigation. All the tanks, honestly, have some capacity for party healing and mitigation (WAR does as well), but PLD has the most on-demand between Clemency, an extra raid mitigation, and the rare uses of Cover. No Raise for a "second healer". Its healing is a DPS loss, to the point it's rarely used without condemnation.
DNC - More a buff focus than a healing focus, but with some healing abilities. But its healing abilities weaved within its damage formula but also gated by long CDs preventing any kind of sustained support healing. No Raise for a "second healer".
RDM - Potential for sustained party healing, but the healing is more or less as weak as it can be and not be
completely trivial. Best Raising potential in the game. Its healing/Raising is a complete DPS loss, however.
SMN - The only other non-healer with a Raise, but its healing outside of Phoenix once every 2 minutes is less than trivial. Phoenix healing is enough to be significant, but is time gated and cannot be held for situations needed, making it often useless overhealing.
We don't have a Healer system with this basis. Although here we could note that other than PLD (for rotational issues), the other three are widely loved classes, and of the people who do love PLD, one of the reasons is because of that support capability. SMN is the most played Caster (and possibly the most played DPS Job), RDM is still very popular, and DNC is the most played Ranged.
I'm not saying it's what would be used for SGE, but SCH and AST having a kind of hybrid support profile would probably be very popular. In effect, isn't this what most of you are asking for already? ARR SCH? Healer Jobs that maintain a DPS rotation and uptime with some damage ability interactions/CDs and such where most healing is done through oGCDs or ability interactions?
This would make the "Support Healers" more fun (rather than stale) in easier content for the players who are bored now, since you'd actually be playing something like DNC or RDM when no healing was needed, shifting more heavily into your healing tools only when the situation demanded it. I don't know DNC that well, but I know RDM decently.
[EDIT to clarify - I mean "A healer/new healer/SGE/SCH, whatever that played like this"]
I would envision something like imagine if RDM had its present day rotation with a few tweaks. Acceleration has a shortish CD (30 sec, maybe) and can be used on heals. Using it on heals generates more Black/White Mana of some set amount (less than if you were using your DPS rotation, but enough to partly refund GCD healing with damage) than their standard non-acceleration amount (which would be as much as Jolt 2) and make them instant cast. Maybe it guarantees procing Stone or Fire Ready (whichever Mana you have less of), the details aren't overly important here. It gains the abilities Vermedica (a simple AOE heal) and Vercure 2 (a larger single target heal). It would also have an AOE oGCD heal with a CD around 60 seconds (Vermedica 2, perhaps), a single target heal like Lustrate/Durchole with a 60 sec CD (Vergrammaton), a second heal that's basically the same thing, but consumes 10/10 or something White/Black Mana (Ver...lustrate?) (so like SCH with non-Energy Drain healing abilities, optimization would be to avoid using it, but it's there if you need spikes of single target emergency healing; RDM also already has something like this in live as a DPS with Enchanted Reprisal as a movement tool but one you generally want to avoid using unless you have to), and probably some kind of oGCD single target barrier, like VerStoneskin as either a Benison or Aquaveil equivalent (every healer has some kind of Aquaveil at this point other than MAYBE SCH depending on how you want to look at Protraction). Finally, healing GCDs don't break the melee/burst combo. If we wanted to, we could make more oGCDs use Mana or add an AOE oGCD that does or something, but this is just a framework.
So your oGCD kit consists of something akin to Physis, Tetra, Lustrate (spamable with Mana), and Divine Benison, along with, of course, Magick Barrier, which is outright (a weaker version of) Temperance
right now, backed up by a strong single target spamable GCD backup heal (Vercure 2) and a AOE spamable GCD backup heal (Vermedica). And, of course, Verraise.
The gameplay would consist of performing the rotation just like today, and when your Pure Healer teammate needed more support, you could use your oGCD healing tools for spot or emergency healing, and in crisis situations, you could support with Dualcast 2x Medica or Cure 2 spam.
This would be a lot like ARR SCH with a main focus on DPS upkeep with oGCD healing support, having GCD backup heals for moments of crisis. The GCD heals could also be used with Acceleration to generate Mana, or could be used as Dualcasts at a minor DPS loss. Using it as the lead cast of a 2 spell pair would refund some of the damage lost by generating a Jolt 2's (spell you want to avoid/"consolation prize" rotation ability when you don't have Fire/Stone anyway) worth of mana. Still a damage loss vs Jolt 2 (what with not doing damage), but wouldn't interrupt your rotational flow if you're able to heal in this way - remember that being one of my considerations for the DPS minded healer, that their rotation flow be as little disrupted as necessary by healing needs in a general sense outside of emergencies.
Surely that wouldn't feel stale, especially since you were just mentioning how stylish and fun RDM looks?
It would still probably be looked at as inherently flawed by many dedicated healing players.
That's the point - dedicated healing players would, instead, play the other Healer subrole, let's call it Dedicated Healing (DH vs SH) to avoid confusion with the current "Pure" or "Barrier" names.
Again, just like in ARR where dedicated healing players didn't enjoy SCH and so played WHM, under this model, they wouldn't play the Support Healer unless they wanted that playstyle. In ARR, if a dedicated healer picked up SCH, they either adapted to or it swapped back to WHM when they realized Adlo wasn't Cure 2 and Succor wasn't Medica.
...I also find it a bit confusing you're arguing against this. Isn't this what you were asking all healers to become? o.O
The game would also need to output damage constantly in order to justify the healer role and not have the meta kill it for a 2x support scenario you mentioned,
P5-P8S say hai.

Not trying to be snarky, just a smidge of humor. We just saw this, so it's clearly not a "change" here, it's what we have right now. It's also what we had in ARR and HW. I think you said before you'd been playing since then as well. Do you remember back when boss autos were actually dangerous AND could occasionally crit? That makes P5S's bleed look tame since at least it's anticipated and controlled.
and who knows how that would impact MSQ instance content.
It wouldn't. As you guys are fond of saying in your idea to change all the healers, MSQ would remain unchanged for the casual player so Healers not into that could play as they do today. No reason to change that.
Likewise, in casual 2 healer content, 2x Supports would work entirely fine. For example, in 24 mans now, you don't need a Pure and Barrier healer. You arguably could probably solo heal all those fights if your party members are avoiding avoidable damage. So for the purposes of ques - 24 mans, leveling dungeons, and so on - there would be no healer shortage even if the Sustain Healers were unpopular (they wouldn't be, btw), since the game could just grab Support Healers to slot in and have no problems.
You'd probably just have a different game entirely at that point.
P5-8S are in this game right here. And again, ARR and HW. I'm not sure it would be "different game entirely" nor "entirely alien" since it's both been this game in the past and is arguably this game this tier.
The reality is, that idea of healer is one that is dying because it was historically very unpopular in older MMOs
Not really. While it's true that some less players want to play them (hence splitting the role so you only need half as many), healers were traditionally more played than Tanks, and still are in a lot of games. Even in FFXIV, Tanks and Healers are roughly equivalent in number of players, and this is with the healing model being unsatisfying for many. So it's never been historically
very unpopular, and has often been more popular than Tank. Still arguably is except in games that allow for a Support role. Note also that Support is something a lot of players enjoy, despite games not having it as a fleshed out role.
Non-scientific, but in polls of the Pantheon community, the split is roughly 18% Tanks, 22% healers, 30% Support, and 30% Damage players. Not exact, but Healers are a bit more than Tanks, and the DPS players split. In many games (no Support role), those players are forced into one of the other three roles, and tend to play either Healer or something like DNC/RDM and find themselves dissatisfied because it's not QUITE what they like best, instead preferring something like a Mesmer/Enchanter/Bard (Everquest type, not FFXIV type). Which is to say, there are a lot of players that genuinely want that playstyle and are kind of just left in the cold.
It turns out a significant amount of people kind of DO want to be a party cheerleader, just a lot of them want to be a different type of cheerleader.
Moreover, I don't think Dedicated Healers are nearly the vanishing base of players you do. I know players who ONLY play healers in games, and they don't DPS. You guys talk all the time about how many 0 DPS Sylphies you run into in all your dungeon and Party Finder Raids, right? Someone earlier said she ran into a healer that ran away from the enemies so they didn't damage any with Assize. Clearly there are a lot of these players (by your collective anecdotes) rather than it being a dying or vanishing playeerbase. If it was vanishing as you say, then you guys wouldn't have all those stories of the 0 DPS healers, right?
They want to engage with the gameplay, just from an angle of someone who prioritizes support and needs to make decisions based on the situation their in of when to attack, when to heal, when to support, etc.
At the risk of taking us to the unproductive place we were a few pages ago - I think here you're stating your own opinion as if it is shared by everyone. You say "They want to engage with the gameplay". Two issues with this are (1) Dedicated Healers ARE engaging with the gameplay, they just aren't engaging with a DPS rotation and (b) why is "They"? I get that "They" includes you, but are you of the mind that you can speak for the entire healing playerbase across all of gaming? Is that not a bold claim?
that these are suggestions would work within the structure that this game provides.
Ahem...
...many are
not. For example, your two concepts (particularly the first one) above would not work with the current game encounter design and party structure as we have it today.
Moreover, what I proposed actually would work just fine. For non-Savage content, it would be irrelevant. For Savage content, you could probably brute force it just like people were able to get a single healer Ultimate clear. Dedicated players could probably 2x Support Healer it if the party as a whole made some modifications (DPS with more healing/mitigation utility) and played well (no taking avoidable damage at all), while the majority would opt for taking one of each healer. It would be no different than P5S if you had one Healer focused on healing all the bleeds while the other Healer focused on DPS, shifting to support healing during the heavy damage phases.
The game is already working like this in the current tier, and did in 6.1 (selfish WHM's forcing their co-healer to do 80+% of the healing is a perversion of this idea, and it literally worked - people hated it because it wasn't intended and designed for)
So I'm confused why you're saying this idea would require a "ground-up rework of the game" considering
both of the previous raid tiers (both in the EW expansion) have had examples of this in the live game (clearly it hasn't required a total rework!), and the game also has this as the roots of what we have today, being the way healing worked in ARR and arguably HW...
I'm a bit confused how you're thinking it requires entire reworks when it's already what we have in live?