Realistically, there are innumerous ways you could go about addressing the problem with healer design and healer engagement in this game. The only limit is the imagination of the designer, and while not every solution will work, there is not only one right answer either, but the key is in identifying which ways will make the largest amount of players from all sides happy.

The thing is, there isn't just one problem area that's causing all this unrest. Healer dissatisfaction stems from a number of weak points in this game's design as a whole:

1. Fight design at all levels does not deal damage frequently enough to require healers to heal more than they attack.
2. Healer kits have become too powerful. Fight design has not adapted to the throughput that the healers are capable of.
3. When no healing is needed, healer gameplay is atrociously slow, boring, and also horribly designed to support a "solo-play friendly" design philosophy.
4. Non-healer sustain is too frequent and too powerful that it lowers the value of the support that healers are meant to bring.

Resolving the issues with healers cannot adequately be corrected by addressing only one of these aspects. The best way to turn the healing role around is to address all of these points within respects of one another.

Now, addressing all these points at the same time is possible, but can also be tricky depending on how they're executed. It's entirely reasonable to say that the best way to approach these points is to do so gradually in waves. For example, perhaps you start with the healer kits, let the community become comfortable with those changes, and ramp up healing requirements across all difficulties of content over the course of an expansion until you find a healthy sweet spot. Meanwhile, non-healers can have some forms of sustain, but perhaps we make it more about utility that empowers the healers rather than doing the work for them.

With WAR, for example, maybe instead of being a flat heal, Equilibrium instead increasing the amount of HP restored to the WAR via healing actions. Overhealing received extends the duration of Thrill of Battle's HP increase to a maximum of 10 more seconds. Shake it off increases the value of the next barrier received on the party by 10%. Things like that can help make that utility about the healers without gutting the tanks or any other job.