WHM would need significant changes to get it up to speed. It would basically need a total top to bottom rework. For starters, introduce an ability called Cleric Stance. This stance doesn't work like ARR/HW Cleric, though. It turns all your heals into damage spells. So under Cleric Stance, Cure is now Stone, Medica is now Holy, and all your HoTs are DoTs. So Regen becomes Aero, Regen 2 (a second HoT with a 2 sec base cast time like old Aero 2) becomes Aero 2, Medica 2 becomes Aero 3, Cure 3 becomes Holy, Cure 2 becomes Water (we'll get to that), and Asylum becomes a damage field like DoTon or Shadow Flare. The only spells not affected are Assize (which already does damage and healing), Raise (which works normally no matter what), and Benediction (which will always be a 100% full heal to a single person), and Solace/Rapture would be unaffected (but are also now oGCDs, so like how SCH would use Lustrate under Cleric in ARR and HW). The stance has a 10 second CD so using it at the wrong time would be punishing since Healers should be aware of boss fights and know better than to use it right before large healing mechanics. It should actually lock you out of all healing other than Assize and Benediction (since your heals turned into damage spells) until you turn it off. Some more button bloat, though Cleric Stance should actually mitigate this, and more variable gameplay for this to work, as well as more things to track.
This consists of at least one more timed CD self-buff (probably with a 90 sec duration to make it not play nice with other buffs and not be easy to remember or track when it's coming off CD, think like how NIN has Mug every 2 mins and Trick Attack every 1 min), at least one more DoT, probably two, and with none of them having durations divisible by 30 seconds, so one 15, one 18, and one 24, increasing the GCDs per minute spent on non-Glares for optimal damage output. A GCD damage spell with a CD of 21 seconds to further mix things up, Water element spell line (so Water, Water II, Water III, Water IV, Banish, etc), with a proc based chance of DoT ticks reducing its CD, and where using it has a chance of making a Thundercloud proc for one of your DoTs (or their DoT ticks can do this themselves, either one would work here; either Water is doubling up with Sharpcast or DoTs are doubling up being Mage Ballad and Thundercloud procing, both are good options). This would further cut into Glare's monopoly on the GCD and do so in a way that interacts with the rest of the kit and has other parts of the kit interact with it. Also an AOE DoT or DoT spread mechanic, and probably some kind of 1-2 rotation where the -2 is only generated by procs (like old MCH Heated Shots), where you only use it when it procs as it would be a DPS loss to use the unbuffed version, and rotates through Quake, Flood, and Tornado each time it's used, with each applying a different debuff and being AOE; Quake would be root, Flood would be Droopsy DoT, and Tornado would be vulnerability up. These effects last for 8 seconds when triggered. The -2 can be proced by both Glare and Holy, and act as a Combo requiring a Glare or Holy to be cast again before casting them to achieve their added status effect (just casing them directly only does the damage), and the cast time is base 3 seconds, longer than the GCD so a bit more like BLM big spells or SMN's Ifrit/Garuda cast time spells. Also removing Lily heals from the GCD and making them oGCDs instead, and giving Misery a chance to proc from...something, maybe Glares, which is your filler cast when Water/Banish is on CD and all your DoTs are already applied and ticking, sort of like Heavy Shot for BRD.
SCH basically give it back its SB toolkit with the addition of Bio as a separate spell line from Biolysis so it has 3 total single target DoTs and Shadow Flare from ARR/HW (can't use at the same time as Sacred Soil and no CD so you can GCD cast it whenever you want for uptime, 15 or 20 sec duration field damage effect like Doton). The DoTs all have different CDs, 15, 24, and 18 seconds, and three spell line to carry us forward keeping it up into future expansions, Bio which is instant cast with the short duration, Miasma with the long duration but being a full GCD cast time spell (2.5 sec), and Biolysis being a quicker 1.5 sec base cast time to upkeep as well. Also give it Fester, the Old SMN version where its damage is based on how many DoTs are on the target, and this will include Miasma 2 and Shadow Flare as separate DoTs for the purposes of its calculation, so up to 500 Potency of spike damage. Also make Selene and Eos their SB forms again. Remove Expedient, because it makes encounters too easy. Add "Martial Stance" that works like ARR/HW Cleric Stance, swapping your Mind and Int stats and reducing the potency of non-Faerie heals by 20%. Make Aetherpact a toggle with no Gauge Cost (remove Faerie Gauge completely) to replicate ARR/HW/SB SCH Embrace macros (alternatively, instead of a toggle, it could be just a command to use Embrace on a given target; both ideas work fine).
AST give it all of its HW and SB and ShB card effects. Return Seals to affecting Divination so people actually have to do that crazy ShB opener if they want the full +6% party buff. And give them a second DoT with a cast time like Old SCH Miasma or WHM Aero 2. Also give them the old Sleeve Draw, Royal Road, and Spread (stock a card to use later) abilities, as well as Time Dilation, Celestial Opposition (when it was AOE Time Dilation), and return Nocturnal and Diurnal Sects to the Job.
SGE give it BLM's rotation. Like, straight port BLM's rotation to SGE but with Greek names for Fire, Thunder, and Blizzard. Kardia is now how they heal party members. All the big BLM spells ( IV versions, Xenoglossy equivalent) generate AOE Kardia pulses, AOE spells like Fire II generate AOE Kardia pulses. SGE doesn't pay attention to health bars, it focuses totally on its DPS rotation. If it does it correctly, the party has sufficient healing to live. If it does not, people KO and must be raised. It only has a very few token healing tools like Durochole that are limited by Addersgall generation rate and are for emergencies only. Give it an Energy Drain Addersgall spender to use Addersgall on (SGE does this wrong, SCH does this right), so when you're doing your rotation optimally, you aren't needing to use Addersgall to heal, you're using it for additional chip damage - if its doing its DPS rotation correctly, it won't need to use them on heals anyway. Very high skill ceiling. Some animations would be reused, but some would obviously need to be added to have the desired effect. Eukrasia could be used as a stance toggle kind of thing for one or the other (the Umbral Ice or the Astral Fire type spells) to save some hotbar spaces and to keep it from being a carbon copy of BLM too much. Basically, the goal here is to make SGE do for Healers what GNB did for Tanks - give DPS minded players (though in this case Casters instead of Melee) a sort of island/oasis Job within another role (for GNB, Tanking; for SGE, Healing) that they could swap to and play. I suppose we could do something more like RDM or even Old MCH (SB days), but BLM would probably be the most directly transferable to a Healer role since it shares the cast time turret general functioning that Healers do. And if doing the damage rotation resulted in party healing with just a few suboptimal to use backup heals with Addersgall, then you have new and complex SGE.
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That's how I would make Healers as needlessly complex and difficult as possible.
Added WHM Cleric Stance and SCH Martial Stance effects.