If I were to make a change to healing in general, I would add something like "Bleeding/Injury".

Basically every time you recover max HP worth of HP you lose 1k HP up to 5 times for either 15-30 seconds.

This would allow healers to continue to do what they do but the fatigue would kick in to those they are healing where they would either end up dying of being forced to pull back for 15-30 seconds to recover. One other thing to add to that, the debuff refreshes if you continue to heal or take dmg so at some point if you want to recover you would need to stay out of combat to fully recover.