You can add more offensive spells and abilities to creates a small, but interesting offensive gameplay loop for all healers, which addresses solo content as well as healing downtime.
You can prune the bloated healing action library and create a smaller, yet also more interactive library of healing actions that are both easier for less experienced players to keep track of and more compelling for skilled players to take advantage of.
You can gradually increase healing requirements from fights of all difficulty modes to try and create more reasons to heal without making it too difficult for the game's target audience to stay in control of.
All of these are independent solutions that, on their own, can improve the situation to some degree, but you cannot solve the problem of one only by addressing the other. Adding more DPS will not change the lack of healing gameplay, nor will it increase the opportunities for healers to engage with healing. Pruning and revamping heals will not fill the gaping hole in healer gameplay when healing isn't needed, nor can it increase how frequently you'll need to heal either; if you weaken healing throughput in an attempt to create a greater need to heal, you risk making old extreme and savage content considerable harder, or even impossible if you don't have the throughput to keep up with that. Increasing the frequency of healing required, but it's not plausible to do this enough to justify the current healing kit bloat, nor can it be enough to eliminate healing downtime as an issue. Resolving one concern and not the others can certainly help, but it won't relieve the role of all of its issues, and may not be enough for some people who have left the role, nor enough to keep some people who are considering leaving the role.
Naturally, the best solution is to address all angles if the issue. That's a pretty daunting task that would require a lot of work. I believe it was Aravell who mentioned how starting with DPS could work to help alleviate some of the tension in the meantime, provided that other changes were also along the way. If they came out and addressed the concerns while providing a roadmap for how they plan to address them, I could be entirely content with that if I agree with their projected endstate.