I think this story is an allegory to some sequence of events that happened in Natanatan at some point. I'm going to be describing the story on two levels - on the level of Cultivation, Honor and Odium, and on the level of Nightwatcher, Stormfather and BAM (who is not the Sibling). The earlier parts of the story fit the former set better, the latter parts the latter set.
The Moons of Roshar are Nomon (brightest, blue), Mishim (slowest, Green), and Salas (darkest, Violet).
Of note are the colors that Hoid uses thoughout. So far, for the Rosharan Shards, the colors that can be best associated are White-Blue for Honor (Stormlight is white, Honorspren are Blue), Green for Cultivation (the mist at the Valley), and Yellow-White for Odium. (Voidlight is technically Stygian Blue, which is a fatigue response to bright, intense yellow on black - most probably this fact is a commentary on the nature of Rayse-Odium)
Let's jump straight into the story then, shall we?
Subtext: Cultivation is cleverest of the three Shards on Roshar.
Or, the Nightwatcher is the cleverest of the three spren the moons are most likely to represent.
Subtext: Honor and Odium are content sit in the spiritual and manifest avatars, powering the magics of Roshar through their respective lights - Stormlight and Voidlight. Honor has the Highstorm, and a high place on Roshar was called "closest to Honor", whereas Odium is literally on a different planet, and now has the Everstorm. As opposed to these two, Cultivation's influence can be seen in all the flora and fauna of Roshar, and she takes a more hands on approach, if only through the Boon-Curse magic.
The reign in the sky could then be a realmatic metaphor, with Honor and Odium always staying away, whereas Cultivation being super invested in the Valley, and personally taking interest whenever she could.
On the Nightwatcher level, we don't quite know the nature of the Sibling enough to be able to relate it like this with the sky and light. Indeed, I've been suspecting the domain of the Sibling is "stone", or Crem, so I don't know if it would hold with the reign in the sky. I've been suspecting that BAM is to Voidbinding what the Stormfather is to Surgebinding, and in that case she could be the sister to the Nightwatcher who is also content to stay away and "grace the lands with her (Void)light," whereas the brother, Stormfather, would also be cotnent to drive the Highstorms and "grace the lands with his (storm)light."
I don't know what Hoid specifically means by this, other than maybe a sly referral to why Cultivation is training the Nightwatcher to be her heir, of some kind.
Yep, Mishim is the green moon, and green is the color most associated with Cultivation, and by extension the Nightwatcher.
Take the towers to be an allegory to something else, say... Ancient Fabrials. The Natanatans would, in this case, be leading scientists, trying to maybe autopsy and study the biology of Greatshells to understand how fabrials would work realmatically. The Natan people were trying to make artificial Fabrials, and impinging on the domain of Cultivation - whose system is this integrated Fabrial magic in the ecosystem of Roshar. It also makes sense in a different way, because the Aimians would eventually be providing Soulcasters to the rest of Roshar, and the end of this story strongly indicates that the Siah were born as an outcome of what happens here later.
The towers could also be an allegory to a lot of different things and not just Fabrials. If you consider the sky and the lands to be the Spiritual and Physical realms, then the tower would be some kind of invested constructs - the metaphor relies on the same description Brandin has previously given to perpendicularities, something that is "heavier" than normal in the physical and stretching it a bit to reach into another realm. A tower is something that is built on land but reaches for the sky.
Translation: I am mortal, my domain the Physical and Shadesmar. The Spiritual is yours. Divinity, yours.
Interesting use of color here - why the white pillar? The description is similar to the description of Honor's perpendicularity that Dalinar summoned at the end of OB (to be fair, they're both described as white pillars, which isn't exactly a spectacularly detailed description, but still), but I don't know if that's what Hoid is actually alluding to or not. Also, why would you need Honor's perpendicularity to ascend to Cultivation?
Except, the allusion is to the fact that Bondsmiths can do it, and this can be used for specific purposes. The answer to that would of course depend on whether the ability to manifest Honor's perpendicularity was a thing Dalinar could do because of the special circumstances involved with Honor's remnants and the Stormfather. Even if it was, that particular ability could be achieved by other Bondsmiths, just not necessarily in that manner, or by those circumstances.
As always, Cultivation was using her Futuresight and her intent/motivation to see how best she could... well, cultivate. Her intent dictates that she constantly be doing that - constantly be looking to cultivate. Especially after holding the Shard for as long as she has, she would be reaching for some Preservation or Ruin levels of singlemindedness. Unlike Honor and Odium, she rarely makes an active play in events, preferring quiet nudges that may yield results favourable to her sometime in the far future. To mortal eyes, this isn't always apparent, her changes may not always be individually beneficial even, and because she uses the Nightwatcher as a substitute, she is culturally characterized with the attributes "loathes her duty" and "lazy". Another element at play here is that it was Honor that was pro-humanity, while Cultivation's own feelings towards them can, at best, be described as lukewarm.
Take this from the perspective of a Shard - for whom looking at all of this in the physical would be trivial. Even Odium, who - unlike Cultivation - is actually trapped, can show Venli visions of Roshar from space, implying that is a perspective a Shard could see through if they so wanted.
Also, yes, if you looked at Stars up close in mortal form, you would truly burn away from the heat of it.
Another thing is that Stormlight infused Gemstones glow, and that perfectly cut Stormlight infused Gemstones would glow for a long time. So yes, without explaining the deep physics of everything, in a story, to say that the Stars are gemstones cut by the finest of gem cutters wouldn't be exactly be an inaccurate metaphor.
Take the Heavens here, to be a state that you attain be Ascending. Yes, the sheer amount of information you would parse then would drive you mad in your non-ascended state. Moreover, as a scholar of Realmatics, Queen Tsa would know this.
No clue what the Starspren mean though.
Whatever these towers represent, investiture is definitely a big part of them.
Or it could be taken figuratively in a different manner - the societal pillars and domes of that city could literally hold membership with the Knights Radiant - they could literally be Surgebinders, and thus Radiant.
The Yellow-Gold smoke is interesting, as Yellow-Gold is the color of Bondsmiths. The green involved could imply this is a Nightwatcher Bondsmith.
The Queen was Pious, which is a Bondsmith trait.
And the Shard of Cultivation, or the Nightwatcher herself, is described as crafty. Also not an inaccurate descriptor.
They're trying to achieve something by Applied Realmatic Theory, but something is missing. They're missing a clue. Something in their equations is wrong. And here they try to convince a Shard to help them.
Queen Tsa asks to ascend - to pick up enough of the Shard to be capable of achieving what she wants to.
Whether this is something Cultivation also secretly wants to come about, or whether this is really Queen Tsa tricking Cultivation is - atleast according to me - a question the story does not have an answer to.
This is probably over analysing (well, technically so is the entirety of this post :P), but why the Orange? Why a random fourth color?
I have only one unlikely conjecture - Odium does appear as the entire spectrum of Yellow to Dalinar in one of the Visions, so the Orange could be him. The fact that the word intense is mentioned could be indicative that the Orange represents something related to Odium, if not directly Odium.
The Shard is temporarily passed from one vessel to another.
Or, the Nightwatcher and her Bondsmith somehow swap places. No clue if this is even possible, although theoretically this could be achieved by rewiring one physical body from one Cognitive and Spiritual aspect onto a different Cognitive and Spiritual aspect. Probably unlikely, but not impossible.
hmmm. So after what was ages, Cultivation's vessel, got a chance to be not a Vessel. Of course, she enjoyed however she could.
Or the Nightwatcher is enjoying being mortal.
I do not know what this part of the story means. The Shardic level was working better so far. But the superspren level makes a better fit from this point on. I don't quite know the exact mechanics/events this story is alluding to.
There's something happening here and I don't quite know what. It is however interesting to note the qualities that are associated with each moon - kindness to Nomon (Honor/Stormfather), and thoughtfulness to Salas (Odium/Sibling/BAM).
It's interesting to know that the color of Tsa is White, as opposed to the Green of Mishim, Blue of Nomon, and Violet of Salas. White is the one color common to all Radiants.
Queen Tsa broke her word. Not very honourable. It is also interesting that that idea - the idea of her breaking her Oaths, is what gets Tsa to relent. This could be a cultural folk based decision in the story if it is not alluding to anything. But it fits this subtext surprisingly well.
Roshar does have a significant focus on the ability to "hear" spiritual ideals, like the Rhythms. In that vein, this is interesting.
Blue (and White) have so far been associated with Honor. So this is interesting.
The first Siah? It is interesting that this was a new song.
This is... I already laid out my feelings on this. I don't quite think that Queen Tsa could have tricked Cultivation herself, and this would be a ploy by Cultivation to make it seem like this, but in actuality, the fact is that some of her goals are also being achieved. These goals could be anything ranging from the creation of the Siah so that they could fulfill some specific purpose in the future, to something like letting go of her Shard for a while to potentially delay or decrease the overpowering effect that it's intent could be having in her consciousness. Maybe both, and more. I also don't know what "loss" means to Cultivation, save perhaps a kind of nostalgia on what she had been a long while ago - an understanding that a life like this with Tanavast is what she gave up all those millenia ago.
If however, the Nightwatcher is the one in question, then this kind of makes better sense? Especially the parts where she didn't know mortal life, or the fact that she couldn't know loss. Cultivation would have allowed it in this case because she would have thought it would ultimately be better for the Nightwatcher to understand mortal existence, particularly if she was being trained as an heir to that Shard someday. This could make even more sense if Tsa was a Nightwatcher Bondsmith, and pulled something akin to what Dalinar managed at the Battle of Thaylen City, but different by circumstance, skill, and motive.
A Child whose mother ascended while she was potentially pregnant, or something else was done by Tsa with her expanded understanding, capabilities, and perhaps most importantly, Honor's (maybe via Stormfather) investiture. Maybe the creation of a certain kind of Slivers was the point of the whole endeavor, or maybe the Siah hold a secret in their genetics/existence that Tsa was looking for all along. (by secret, I don't necessarily mean lore, but rather the fruit of Tsa's endeavors - art of a kind)
Read above on what I feel about this.
So... yeah. Had to get this out, though it would have been fun if I could have gotten to this before the Myths of Roshar Shardcasts.