I have thought about this idea as well, and reached a different conclusion. There should not be a healer subclass, since it would be so situational. Instead, for specific encounters that are designed with a healer in mind, they could simply add a healer item that any raid member can pick up. We already have something similar to this in the Crota fight with how automatic healing is disabled and raid members have to use the chalice. A theoretical future raid encounter could have a super shield gun item that you pickup and use to shoot at your teammates to superbuff their shields to survive a specific blast or something, or during raid encounters when automatic healing is disabled. A special tanking item could also be designed as well. The fact that so many raid encounters have unique items (Swords, Relic Shields, Chalices) leads me to believe that items is how they will handle raid healing should an encounter ever require it.