Maybe they should look outside XIV for inspiration for other healers.

They could take the samba system from FFXI's DNC. Apply a status on the enemy, and when people damage the enemy they are healed. This could be used instead of the standard AoE regen spell, but having a downside of not healing when the enemy is untargetable.
Make it a Darkness aligned healer. Give it a 0-100 blood guage that interact with another debuff. When the debuff is applied to the enemy and the enemy is damaged by any party members, it fills the blood guage. Then the guage can be used to power stronger healing spells.
This would create a healer that uses the party to power their healing.

Could also have a healer that uses oGCD-cooldowns to boost their spells. Turn Esuna into AoE instead of single target, or boost Esuna to remove all debuffs from a single target(like a tank getting hit by Bad Breath). Another oGCD ability could be making regen spell last longer but at the cost of more MP, or shorter duration but with stronger healing per tick. The healer have to use the oGCDs strategically for best usage as they can't be spammed.