I'm sure players who actually play a healer knows better than I do since I don't play healers due to them not being my interested playstyle.
I simply only have my personal impressions and what I can tell how fights are while playing my roles.
With that said, I can't see how healings in a set rotation wouldn't end up with simply over-healer or wasting healing skills during moments when no one's hurt, and so on.
Tanks and DPS will always have something to hit on where healers might not always have someone to heal during a fight, so to me a rotation of healing seems kinda wasted and better used as off-rotation skills used when needed.
So instead the easiest solution to me would simply be to just give healers more damage spells to use during the downtimes between healing.
However, that doesn't mean that one couldn't come up with interesting ways on how healing is done.
Like it could be a healing but with an additional buff to an ally.
That way, even if the heal would end up as 80% overheal the additional effect would still be useful regardless of the amount of healing that ended up getting "wasted".
Shield healers could have so that applying a shield even outside of a non-critical attack could have an effect that when the shield breaks.
One could come up with many ideas and I'm sure that players who play healers have far more ideas than a non healer like me could come up with.
But there's one thing I would say that I think many can agree with, is that the different healer jobs lack their own personality.
And if each healer had a stronger personality apart from one another, one could from there also come up with ideas of how to make them more fun to play fitting with said personality.
Like, lets say a Druid in a game, their concept is build around nature so they have a lot of nature spells and rejuvenation spells but also attacks based around nature that can have different effects on an enemy, like slow, blind, whatever fits the theme.
So I think a stronger job personality would help when brain storming ways to improve each of them.