At the risk of sounding like a stuck record...
Warhammer Online's healers were an interesting case study in this sort of thing. Whilst the execution wasn't perfect across the board, the healers were very tightly designed with large amounts of cross kit synergy. With the melee healer you quite literally fed the resource needed for your powerhouse burst and aoe heals by swinging your hammer. The cleric style healer had a complex buff system to play around with. The robed healer was built around the mechanic that the more you DPS'd, the more your heals were enhanced whilst the more you healed, the more your DPS spells were enhanced with the goal being to try and maintain a median to get the best results.
Again, it wasn't entirely perfect, the robed healer was arguably the least well tuned of the 3 with it's system not really paying off but I'll argue that for all it's flaws, the melee healers were the most enjoyable well designed and rewarding healing experience I've ever seen in any MMO I've played. It rewarded playing on the edge and taking educated risks like no other healer.
Basically, if it was my task to tackle this issue, my first avenue would be looking at interactions and synergy within our kit or other ways in which good play could actively be rewarded.