Astrologian also has Earthly Star as delayed healing (With the caveat that it can be detonated early if necessary. Including having 2 separate potencies depending on how long you leave it before detonating)

I think the main issue with this type of healing, is one of, where does it fit in?

Like, the main draw from something like this is to gate higher healing values behind a wall that requires preparation and knowledge of incoming damage - Such as a boss standing there glowing and basically saying "Ooooh watch out I'm about to do an AoE!!!!"

To which, Shields already provide an effective form of pre-emptive healing (Especially given their 30s durations. I often re-apply shields right AFTER an AoE because they'll literally last through to the next AoE usage and it means that the minor healing the skill does heals up any damage that made it through the shields)

While at the same time, oGCD heals allow people to react to the damage instantly having functionally the same effect as a perfectly timed delayed heal, only with expanded viability since you can then also use them to react to some unexpected damage too, similar to Shields (Unexpected damage comes in, pop on a shield to prevent further damage, then heal up with direct heals)

Like, given how things work, there's not really much of a need for time gated healing, especially if you have to use a GCD to cast the thing in the first place (At which point, why not simply pre-cast a normal direct heal just as the damage is coming in?)

Which is why the current delayed healing is tied in as merely a failsafe for conditional triggers: Sole Survivor being about getting the target killed, but if it doesn't die you get a reduced heal eventually. Excogitation being a safety net that procs when a target drops low but if they don't then you get the heal eventually. Earthly Star being an oGCD trigger you control, but if you don't press it you'll get the heal eventually.

So I could easily see more of this kind of thing, conditional healing skills that have a delayed healing as a back up plan. Rather than the main focus. For example, shields that if they don't break from damage and time out, provide healing.

As far as pre-emptive healing goes, I feel a better focus would be on triggered healing.

Wherein, like Excogitation there will be triggers for a heal that you applied at a previous point in time. So, instead of having Regen which can do a lot of overhealing if a target isn't taking damage at the same (Or faster) rate it ticks at, you'd have a spell that you'd apply with a certain number of triggers that heals when the target receives damage. Potentially even with a minimum health threshold (I.e. They have to take at least 5% of their max health for the trigger and weaker fluff damage doesn't proc them)

Maybe a debuff on an enemy where after they take a certain amount of damage, procs a heal around them.

You could even link it to other forms of healing such as having a spell that triggers a heal when the target receives a Shield.

Of course, given how these trigger skills work, you could still incorporate you vision of delayed healing as failsafes for if they time out without proccing (Or using up all their triggers)