There are actually a lot of ways we could adjust encounter design to make healer design a more balanced system. A lot of these require some pretty significant changes to how encounters are designed to begin with, but here are some examples, some of which have been discussed in the past on the forums:

1. Replace Vulnerability Stacks with Damage Dealt Down. When players make mistakes, their most common "punishment" is that they take more damage for a minute, sometimes longer. This actually causes more work for the healers and not the DPS or Tanks who make those mistakes. If we switched Vulnerability to Damage Down, then suddenly the punishment is largely on just the person who failed the mechanic. If this means the party fails to meet a DPS check, then yes it's a punishment on the party, but it does make mechanics easier to survive which would actually speed up Prog times regardless since your party would get more time to see more mechanics in a fight and start learning them sooner. This inadvertently allows designers to pump more damage into unavoidable attacks because they'd be able to exclude some amount of theoretical damage from overall calculations.

2. Make more avoidable mechanics about debuffs rather than damage. It probably wouldn't be ideal to make all forms of avoidable mechanics debuffs only, but if we replaced an instance of puddles, for example, with exclusively that damage down debuff mentioned above, it once again is something that punishes the person who failed the mechanic and not the healer. Fully removing damage from those puddles means that unavoidable damage around those puddles can afford to be a lot stronger or more frequent because we know exactly how much damage the party will be taking in that period of time rather than having an amount of theoretical damage that we can't fully predict.

3. Replace some direct damage with cleansable damage over time. This gives healers more of a reason to use Esuna at times, but it also gives healers more time to respond to damage since not all of it is being dealt at once, not to mention players could counter it with regens rather than just using Esuna. Sage's new Haima/Panhaima would be perfect for a mechanic like that, not to mention Kardia, Embrace, Lilybell, Macrocosmos...We actually are getting lots of great tools that would be very helpful against DoT.

4. Doom, specifically the doom from DR. For anyone not familiar, in Delubrum Reginae, rather than Vulnerability stacks you get doom stacks. If you fail a mechanic, you get one stack, and if you get another before the duration runs out, you're afflicted with a non-dispellable doom. These mechanics also do damage, but we could also just have avoidable mechanics that just inflict this doom if we want more severe challenge in completing things. Again, it allows us to make unavoidable damage more frequent and more powerful if avoidable damage is more controlled.

5. Add % damage based on current health to unavoidable damage. Let's say for example that a boss has a raidwide that has a potency of 500 for the sake of this example. Instead, we could lower that to 200 but have it also deal 30% of each target's current HP as damage, not to be confused with max HP. This means raidwides can still KO players at low health, but it counters the issue of gear scaling while also softening the blow to something more survivable at mid health ranges. You could even make some exclusively based on current HP if you want specific raidwides to not be capable of KOing the party. We already have fights that reduce the party's HP to 1 at times. Pair that with max HP doom, and you can make healing more demanding without necessarily punishing them.

I don't foresee all of this ever really taking effect, but there are certainly ways to encourage healing without scaring off casual players since we can customize phases to not actually KO players, at least not initially. Stuff like doom on party members that needs to be cleansed by healing everyone up to 100% max can be scary, but is often accompanied by large breaks in boss damage to give players time to catch up. We could use these types of things more to allow for damage to be more controlled on the design side.