I've been thinking about theorecrafting, and put in a lot of work in a WHM rework, and a bit into a SCH one. The big thing is that I've been using a newer framework, that attempts to answer what the game is testing as a whole, and then asking how to design jobs to match the game. There is the assumption that to be good at content is to be able to consistently clear said content, and that encounters will be designed similarly to how they are now. As such, there are two axis to consider. The game tests knowledge and observation, with knowledge given the greater weight. The players must prepare for these encounters and execute on them, however, preparation is key (a failed run is prep for the next one, here).
Knowledge * Preparation : Fight knowledge, spreadsheet planning, job muscle memory, general game knowledge. This is the most important thing for a clear. Going in blind means you are only bringing your muscle memory and general knowledge to beat, and you have to learn while you fight; gaining information from those who came before will quickly get you to a point where your reflexes can carry you the rest of the way.
Knowledge * Execution : The ability to execute mechanics while doing your job. This is "Uptime". Easier encounters will be lax on this, but this is what enrage timers, above all else, test. Second most importantly, here.
Observation * Preparation : The ability to learn. This is most important for world first races and others who do things blind, but unless you have someone doing all the shotcalling for you, even watching a video needs a bit of this in order for it to sink in.
Observation * Reaction : Dealing with random or chaotic encounter design, as well as players not playing well. The game is currently designed for players to minimize needing this as much as possible. That said, if you do BLU raids or crazy stuff like healerless ultimates, this is very important due to the increased chaos. Plus, recovering from a supposed wipe does feel good, even if rare.
After all of that, I think the big thing is that the healers don't really test knowledge that hard. There is optimization, yes, but only if you need to squeeze as much as you can from your primary heals. My answer is to put "cool stuff" that players may want to do in a job that requires knowledge and practice to access effectively. Or, less stuff gated by only a cool down timer.
As an example, my WHM idea would bring back a revised Cleric Stance. All of your good stuff can only be used in Cleric Stance. Out of Cleric Stance you only have Cure II, Medica, and an oGCD or two - enough to keep people alive in anything Extreme and under. Keeping DPS up and replenishing resources requires changing your stance. But unlike DPS who tend to have tight buff windows for max deeps, WHM is flexible in when they change stance while not dropping damage. And if things really it the fan, you can replenish resources quickly, but it is not as smooth to do so; or you can spam Cures and Medicas, since those work as well as Stone to replenish lilies, of course.
This will allow players to Medica a Expert dungeon raidwide to get people topped, but good play allows them to set up some mit beforehand tied with a regen in a way that requires some forethought outside of "is it up yet".
TL;DR : design should engage the brain even on simple stuff. Do this by making cool stuff take a little work to access.