At the risk of sounding like a stuck record. For all it's flaws (and rapid failure) Warhammer Online is an amazing case study in how to do this both well and badly.
Warrior priest was the gold standard, your DPS literally powered your healing and you had to take some risks with your positioning to be at your best both in terms of DPS and healing. PvP Balance issues aside, I'd argue that it's the most innovative healing class ever devised in any MMO I've played as it's just unlike anything else. The synergy and depth within it's kit was incredible and SE would do well to take a closer look at what made it tick.