This sounds like a terrible idea >_< this would require a ton of work that I'd rather see go into expanding the instances available to run. Contrary to popular belief SE doesn't build these instances like you'd toss together a house in Minecraft...it takes a ton of time and effort to create, QA and balance these places. Besides PRAE groups are generally 1 healer / 1 tank / 6 dps. This is a persistent problem in all games, doing what you suggest would be very little help in completely resolving the unsolvable issue.

*Edit* Better idea, make all content able to be tanked by DPS and healed completely by Pets which everyone gets access to. Solved. *Sarcasm*