Honestly I think they should tie healers as a job to specific mechanics in dungeons and trials. I remember this boss in Tera when healer had to aggro multiple trash packs and run in circles around the arena trying to dodge mobs' attacks while simultaneously healing their party members. This felt very engaging and fun, every dungeon you had to do your specific thing thus making a healer relevant no matter what.

Now, whether it would be considered fun by ffxiv players I don't know, but it's definitely an alternative if we're not trying to change healers as a job. Sure, you can have your one button dmg spell and 2 buttons for healing, but you also have to do this specific mechanic that nobody else would do.