For both the above and below healer concepts may introduce capped interactive skills. An issue Astrologian had when they could buff an entire party was 5% on 8 was worht more than on 4, and on top of that depending on what job and who you were with it could be even more inconsistent. Potentially can avoid that by saying "10% extra damage up to 500 potency". "Deals 100 potency per attack up to 500". Etc. Etc.
I think in my dark healer thread I had suggested hemorrhaging spine (curse themed healer lol) which increased damage by a fixed potency from the rear and ranged up to a fixed value. One potential benefit being given the fixed value, if you make the buffs last a fair amount they can 'tend' to be full value as long as even a few people are trying.
Some other ideas brought up for that offensive healer were monster curse (heals) where you would prevent damage coming from the target (not to supplant normal heals, just a supplement), Damage Pets, Auras with interactions (so they don't just sit on your bar), RoA / DoA (regen / damage on action debuff mechanics, kardia vaguely is like the idea), offensive spell that can be used on allies as a heal, and Gray Health which was simply a way to layer in new healing spells (if you 'prepped' a player you could more effectively return their health).
https://forum.square-enix.com/ffxiv/threads/392280