Well, my first thought is actually that the Forge of Destiny world operates upon Platonic Forms, and qi is derivative of those forms. An interesting aspect of the lecture is that qi "is the true form of all things." This just sounds like something from Platonic Forms, and it might be that there is a possible connection between the two ideas. That there is an ideal form of something that is represented by qi, or that qi is the mold upon which the Platonic forms are created and thus imprinted upon the actual world.
A less outlandish idea is that qi is the building blocks of matter and energy. Similar to how reality has subatomic particles and photons which can act suspiciously like they have mass while also behaving like energy. Qi could be something similar, the gloop from which matter and energy are crafted.
I'm not sure that the Great Spirits have control over the world to keep it the way that they like it, although that is a possibility. I believe that the great spirits that we know about are simply a purified expression of an aspect that is already inherent in the world. Rather than the Great spirits controlling aspects of the world to match themselves, the Great spirits exemplify aspects that already exist in the world.
And this is why I believe that viewing the FoD world through the lens of Platonic Forms has some merit. That there are forms inherent to the world that cultivators are trying to exemplify and the closest knowledge we have of doing so is becoming a great spirit. This idea has problems, however, in that to my understanding of traditional idea about Platonic Form is that there is a finite number of such forms. With the number of cultivators over the years and the impossibility of completely copying a domain, it seems like either there is an infinite of the Platonic Forms or there is an infinite combination of Platonic Forms.
The idea that Platonic forms govern FoD really suffers, however, with the differences between the cultures and how they treat great spirits. In the Pillar-Men culture, for example, the Sun is much more dangerous and deadly than in the Empire, where it is more benevolent. This indicates that the environment of the culture actually affects how people treat and deal with Great Spirits and how the Great Spirits deal with them if they are the same at all which is a questionable proposition. These discrepancies with how the Great Spirits act between cultures and environments supports the idea that the Great Spirits exemplify aspects in the world, but that Great Spirits are more local and are actually exemplifying a local aspect of the world. Which kind of thrashes the idea of Platonic Forms that could be universially applied to the FoD world.
I like this and it is a very interesting idea. The Platonic Forms in there basic level are a philosophical outlook which splits the essence of a thing, something similar being referred to in Catholic theology as "substance", and the physicality of a thing i.e the thing you hold in your hand, which was referred to as the "person" from what I recall.
We know qi can take on shapes and that those shapes are given associations, Lake and joy for example. I'm interested in the idea of qi acting as a mold for ideas of an essence like nature like the Forms but I'm not sure where to go with that.
Anyway the idea of qi as the primordial gloop would basically fit with what Hua Heng is describing. There is a question that comes up here. What
is qi? What is it's expanse, depth and measure? So far we've described it as
energy and that hasn't been contested by anything in the lore of FoD or in the lore of Xianxia in general where it is described as a motive energy or life energy or Force of Being if you will. It is inherent to things, in the sense that a rock can have qi sitting in it just
there existing, and then the rock is itself claimed to be made up of Qi.
This line of thought makes me think of the Prima Materia idea of Alchemical and Gnostic fame.
Fun speculation tangents we can get to at some point.
On the nature of Great Spirit's interactions for the material world. Something to chew over is that by information we have been told that Domains are utterly unique, and they are the root from which grows the Great Spirit. So by that measure the idea that people are wholesale
creating Platonic Forms out of nothing but their own heads occurs to me as possible. (I'm not challenging your position, that's not the point of this little speculation fest)
Spirits exemplifying certain aspects of a world would be something interesting because you get this chicken and egg question of "What made those aspects?" which the immediate answer for is the Mother and Father and that they contained everything within them. Which leads into the idea that cultivation is actually a Time of Troubles esque resurrection bid to reforge themselves out of all the separate Great Spirits who are their descendants after said descendants get done filling out the metaphorical coloring book of aspects. Unknowing Fetich Souls yo.
Coming back down from that particular high peak of What? there's your point about the lack of
universality for the Platonic Forms idea you had. There
is this sort of locality to the Forge world(I'm gonna misspell that as Forgeworld at some point and make an inadvertent reference). Perhaps what it is is that Great Spirits exemplify an given aspect of a
given culture. Some cultures are going to have less occurrences of one aspect vs another, like your hostile Sun and benevolent Sun comparison and that could tilt what cultivators go for.
The use of local Platonic Forms could be done because as we know most Way's are not singular concepts, sort of. They are singular
composite concepts, made out of lesser and simpler concepts. Like Emperor An becoming Inexorable Justice, a specific facet of Justice, modified with the concept of Inexorability(Dude musta been mountain as fuck by the way).
Another thought I am considering is that the resting state of the Forge world is chaos of shifting qi environments as described in the past, which is a line of thought with it's own questions like "What supports that idea and notion that the primordial chaos is the 'simplest state'?", where you have lakes of fire, spires of flowing earth, and lakes of solid wood or something really weird. Faerie lands basically. The thing here is that there basically
is no unexplored land where this could happen. Hell we don't even know if this world is a
sphere. Could be there's a boundary somewhere and when you get near it your presence tows along your associated Great Spirits and forms something out of the nothing beyond the boundary.