If you boil it down to its essence, there is one problem with healers: They spend most of their time pressing one button. There are so many ways to fix this it's incredible and honestly really simple. To make a quick list: Do what WoW did in 10.1 (increase enemy damage and player health by a lot so healers can't top people off as easily), give healers rudimentary damage rotations, make healing spells more in depth so you can't just spam medica/succor, make regenerating MP more complex instead of just hitting lucid dreaming, make raid damage way more frequent like in Barb, put most heals on the GCD so using GCD heals doesn't feel terrible (this is probably a terrible idea but an idea nonetheless), the list goes on and on and on. At this point I borderline don't care what they do so long as they do something to make them more engaging.