I'm just going to throw in my opinion as someone who's been healing since Stormblood. I think the point of the healers not being unique enough is fair, however, I have become more and more in love with healing, and Scholar, because of the more recent changes to the job and healing in general. Most of my examples will involve Scholar as it's the job I have the most experience with but I believe the general idea of my points can be applied to the other healing jobs.

I personally don't think much needs changing and if anything, fleshing out the identity of each job's unique aspects should be the direction. Adding more to spend the fairy gauge on, for example, like damage spells, buffs and more utility based abilities. I also wouldn't enjoy Scholar if things like Miasma and Shadow Flare were brought back, I think they are needless and bloat the job too much. I'd rather focus on managing my Fairy's CDs with the shielding than having to re-apply dots. Although, Shadow Flare's effect could be added to sacred soil to give it a dual purpose. I do want to add that I agree that the damage "rotation", if you can call it that, for healers is underwhelming and most of the time a little boring, but again, adding the damage aspect to the identity of the job would benefit it more than just adding more spells to cast. The idea of merging with your fairy to cast more powerful damaging spells is something that I was thinking of as a general suggestion

The stage we're at right now is one where healers would benefit from new ideas going deeper into identity rather than simply adding things just for the sake of it.