I do remember when we did have Cross Action skills. When I was leveling Tharmaturge, I was using Cure during the Umbral Ice Stance on others doing a FATE boss with me. Course now, it is not very viable to do for a BLM if even I don't acknowledge the spell as a 90 WHM. It probably could have had interesting interactions, but it got removed due to some players feeling forced to level a class they don't want. Plus it would be some interesting balancing needing to have 2 different versions of the same spell.
I was mentioning the cellphone game just in case it might have some ideas that the 14 devs here might find possible. It would require re-adapting though since the cellphone one has something like 6 unique attacks that each character has at least. Sometimes a couple more if a Burst aura or maybe an LD attack puts up an Overhead buff to change the base Brave or HP Attacks. As a result, most of the healer support have to have healing coupled into their damage spells due to the small spell space available. Yuna has a couple extra BRV and HP Attack conversions for using her summon attacks since Skill 1 and 2 are Esuna and Cheer. There was usually a niche or 2 each healer had to make them special for why you are bringing them. I could name a few niche parts of each to see which ones might work here.
Rosa - Auto-life Reraise buff. As I said with Japan's rework, she has higher personal damage.
Paladin Cecil - He is basically a kitchen sink buff bot including defenses too.
Ceodore - A few limited LD debuff cleansing with a full heal packaged with it. He deals more damage when everyone is full health.
Leon - enchants the enemies to deal dark damage with their non-elemental attacks and makes the party absorb the dark element.
Minwu - Applies a trap debuff that attacks the enemies whenever the turn counter on it decreases. The trap counters heal the party. LD attack applies overhead that also triggers Sublime Soul off turn attack when the turn counter reaches 5. Turn counter increases from each trap effect and refreshes the trap debuffs to 6 when Sublime Soul triggers. To top it off, he enchants for Holy for the party.
Zidane - His burst aura allows his attacks to "steal elixirs" to pretty much full heal everyone. He can steal enemy turns when brave breaking them and act again.
Garnet - Eternal Darkness Off-turn attacks with Water and Thunder enchants
Eiko - Phoenix Raise cheesing as long as you use her LD Phoenix summon for 3 Auto-Life overhead buff stocks each. HP regen and Holy enchant on the side too.
Freya - HP regen with Jump. Last stand debuff so party members keep 1HP as long as they had 50% HP prior to an HP attack. The Jump is actually much more useful here allowing her to do off turn attacks per action until her turn comes up to sail back down. Her off turn jump attack also heals.
Yuna - she is the standard debuff cleanser and provides a bit more personal DPS
Kimahri - LD HP Regen with a decent heal packaged with it. His EX blinds enemies with 100% hit rate down. Bad Breath reduces HP damage enemies deal when EX is not up.
Porom - raises everyone's max HP and make everyone harder to kill. Has HP based off attacks too.
Lenna - also raises everyone's max HP and provides 100% debuff evasion. Her LD is Party HP based for damage done to completely ignore enemy defense.
Locke - Phoenix Aura just does a standard heal. He has a changed HP attack with chunky damage. Burst aura allows everyone to ignore defense and keep everyone healed to 50% as long as the HP attack does not kill them.
Mog - HP Regen. Uses El Nino follow up attack to clear debuffs on party if any are on them. Uses Plasma instead to confuse enemies if party has no debuffs.
Aerith - Has a Seal Evil debuff to shut down enemy buffs and Debuffs sent to the party. Holy off turn beams and 100% damage reduction for the party for a short time. Not used much, but she can also resurrect a party member about 4 times.
Cait Sith - HP Regen. LD instantly breaks all enemies regardless of brave number and grants brave regen on all brave attacks party members use.
Rude - heals for a high amount and converts overheals to brave gains. LD makes it guaranteed for him to deal critical strikes if I remember right.
Selphie - increases recharge time of other EX abilities with Aura and Rapture debuffs enemies to always get launched into the air by any attack.
Rinoa - she is more of a nuke HP Attack DPS and can be a very good AoE Damage Dealer. She has healing in her kit just in case her self damaging attacks bring her party down too low.
Aphmau - Has LD turn manipulation. 1 ally turn is pulled up to her turn.
Arciela - One button can cleanse the party with another one debuffing the enemy to prohibit any HP attack usage.
Penelo - Has HP regen and a party debuff cleanse. Her burst aura provides Brave gains for other party members based on her maximum possible HP damage cap. Her HP attack is changed to do chunky damage while the burst aura is up.
Llyud - at the time of his release, he had very high Attack buffs and did decent personal DPS. Has high HP, but some attacks sacrifice his health which was only useful when a boss mech wanted us to damage ourselves.
Hope - your standard Protect and Shell healer while also raising Max HP of the party. He also imperils enemies to be weak to magic attacks.
Alphinaud - pretty much uses the Arcanist spells with Energy Drain healing the party and a carbuncle to deal a little more damage.
Y'shtola - stone can slightly delay a target turn with Medica heals. If Medica is not needed, her BRV attack is changed to Aero for damage. Her Burst aura makes brave gains much more powerful.
Ignis - Fire enchant mostly. He can provide debuff evasion and slight turn manipulation pulling everyone's turns up with the EX.
Lunafreya - she can buff everyone to have 2 - 3 instant turns on any skill they use with 10% more HP damage done. Has water and ice enchant.
Ramza - can instantly break an enemy's brave meter regardless of number amount. HP regen. Burst aura can provide highest HP damage bonus based on buffs party members have. LD allows him to launch an enemy to the air 3 times.
Rem - can turn manipulate at will pulling an ally up to her turn while also speeding herself up too. Has a follow up attack from any action she does.
Deuce - HP regen. Mostly increases Magic Attack Power for English version.
Sharlotta - HP regen. Uses Gravity to easily shave enemy brave to zero, but can have troubles brave breaking with almost no brave attacks. Burst aura has the best buffs to brave gains on party. Has LD attack that provides more decent personal DPS.
Currently for Global English, Aerith is the best choice. Penelo, Garnet, Lunafreya and Sharlotta are decent second choices. Most of the rest may need reworks so their personal damage and utility are up to par for the hardest content.
For Japanese content, Deuce ascended to best healing support currently. Her HP damage bonus aura was madly buffed and her FR attack is instant turn with very good conditions to meet for raising the bonus. Rem also becomes pretty good with a Burst Aura that does not let anyone die even at zero HP. Japan has a few new ones like Iris (FF15) and Selh'teus (FF11) who are stupidly good too. Iris neuters the enemy Attack, Defense and Max Brave to become zero and Selh'teus is more of a nuke HP Attack DPS than healer.
The constant utility I see that is most possible to do would be to create a spell that raises Max HP. Some other property to increase healing received with the raised HP will be needed too so the party isn't just MP sponges with high HP.
Sorry if the editing was becoming very constant. English version of Dissidia FF Opera Omnia has about 166 total characters.