I dont know if it has been said or not, but another issue with how healers feel like they have nothing to do, is how damage is being dealt to the raid/group.

Can you think of any raid wide that isnt just "Deals a bunch of damage that almost kills everyone" or "Deals a bunch of damage that will kill everyone with the DoT it applies afterward" (not accounting for actually using any mit for these). So of course the way healers NEED to be setup is to counter act this nearly instantly, which i think is really basic, and not engaging at all. Having more damage hitting the raid is not the same as damage hitting the raid more often.

Aside from changing the design direction of how healers should work, to keep healers busy we need encounters that have constant damage throughout the engagement. Now, that could be a light DoT applies for the entirety of the encounter, a heavy DoT that keeps jumping to other players, or a myriad of other ideas for constant or randomly-dealt damage.

Unfortunately, I believe its less likely for how encounters are setup to be changed in the future than it is for how healers are set up in the future. And with FFXIV being a really structured game, especially for boss encounters, the ideas behind how bosses are made are likely to never change.