Via Dust, we learn that the Nine are 4 dimensional entities that exist outside of time-as-we-know-it
We know that "time-as-we-know-it" has been a dimension of existence ever since the wager between the Gardener and the Winnower spilled outside of the Garden. A corollary to this is that should these two ever rejoin again completely, we can likely conclude that would be the "end" of time-as-we-know-it.
We know that one of the few things which can cause time to get funky is the collision of Light and Dark (as seen in the kugelblitz that birthed the Distributary and the Awoken)
We have strong reason to believe that the Nine, or entites like the Nine, only exist in Sol:
-First of all, they are the only ones we have evidence for
-Second of all, we've seen that when our Nine reach out into the universe to seek others like themselves, they get no response:
>The question remains – in the wrinkled fabric – why Sol?
>[The color fades from sight.]
>II does not see - other seeds along the bend – calls
>Unanswered
>[There isn't an answer in you.]
>Are we – alone
>We – do not – know
So we are left to wonder, what is it about our solar system, and our Nine that make them so unique in the universe? Even if we assume that other planets with sentient life are capable of producing gaiaforms (which is disputable) - clearly something unique about the gaiaforms in Sol allowed them to manipulate events across time such that they didn't number among the billions of worlds shattered by the Witness during its crusade.
What makes our Nine so special? We may have a hint in the first piece of lore they appeared:
Anyone familiar with the lore should have an inkling of what this "Transcendent Shape" is - the union of Sphere and Tetrahedron, of Circle and Triangle, of Light and Dark. Scientifically speaking, annihilation most commonly refers to the collision of matter and anti-matter.
So, lets say that gaiaforms can appear on any planet with sentient life. What happened to burn our Nine into the very fabric of spacetime?
Obviously this is just speculation, and we may never find out, but I think things were being set up such that the Fate saga would have seen the Nine bound to our space-time itself, via the energy released from the annihilation of Light and Dark at the end of time - "burning them in to the weft of what is", which would explain their uniqueness and 4-dimensional nature quite well.
Edit: OH OH OH - AND - lets not forget their recently revealed power: Eclipse
What is an eclipse? well it's when one shape of Darkness covers another shape of Light (a bit like a transcendent shape?). How could entities which exist outside of time have the power to nullify (or, as we are told but not shown, amplify) paracausality? well...what if it's because their whole existence is predicated on it and inextricably linked to it?
We know that Bungie has gone to length to drive home the point that, despite surface appearances, the Darkness is not "morally evil" and the Light is not "morally good". We've seen Dark-aligned and morally evil entities/factions (The Witness, The Hive). We've seen Dark-aligned and morally neutral or even good factions (The Qugu for example don't seem to have been particularly evil). We of course have Light-aligned and morally good factions, who we've spent most of our time with (The Vanguard, House Light, etc). I think a case could be made that some of the Risen/Warlords might generally fit as "Light-aligned, but morally evil", but at the same time I think that given how little was known about the Light and Dark at the time the potential thematic resonance will be a bit limited.
I think the best glimpse we've gotten at such a thing - (Light untethered from "morality" - which is admittedly a bit subjective in the Destiny universe, but I'm pondering this from our perspective, as anthropocentric as that may be!) - comes from the Hunter (Flowing) CODA set:
https://www.ishtar-collective.net/categories/flowing-suit
Where we are told via a vision from the Nine about a world of overbearing LIGHT:
>"What is the Darkness?"
>You open your eyes and gaze at your hands, seeking an answer to your question.
>Searing glow from a tyrant light above you annihilates all shadow from the plain of sand you stand upon in a world full of L I G H T.
>The last thing you perceive is a blazing outline of alabaster fingers gripping your wrist in a tight fist, before photokeratitis takes your sight.
>The roar of the wind fills your ears.
>Whatever has seized you is shaking you. You perceive shouting over the rush of air, but you can't make out the words. You lean closer to your hands, to whatever's clasping them, shaking them.
>The shouting grows eager.
>You can smell it now; whatever has seized you. Ancient. Rotting. Powerful.
>Its grip is strong—as strong as yours, the heat of the Light coursing through it.
>It can smell the Light on you, too. It knows you are just like it.
>It has lived forever. A gift from your shared parent. Forever is too long.
>You think you know what it's saying now.
>It begs for death.
>Your vision gradually returns…
>A harsh glare blooms from the heavens above.
>Your soul is weary.
and in a similar vein, the Winnower gives us a glimpse at a universe where he doesn't exist, back in the first entry of Unveiling:
>But imagine the abomination of a world where nothing can end and no choice can be preferred to any other. Imagine the things that would suffer and never die. Imagine the lies that would flourish without context or corrective. Imagine a world without me.
So we see that Light without Morality takes the form of:
-overwhelming sensation
-no regard for suffering (it's just another possible experience after all)
-chaotic/meaningless growth (i'm reminded of the parable of p53..."A gardener and a winnower collaborate to create a protein. Whose hand is it in the design, that shortens one life to extend the rest?")
Interestingly enough, another long running franchise is about to address this (or at least a similar) question after sowing 20+ years of seeds. In the final WoW expansion, it seems like the World of Warcraft itself (azeroth) will be the object of a battle between the 6 cosmic forces. In that cosmology, the fundamental forces of Light, Life, and Order will battle Shadow, Death, and Dis-order for the soul of the world. Much like the revelation of "morality" in the Destiny universe not being tied to either Light or Dark, a similar thing is happening in WoW, where the long held assumption that the Titans (the "gods of Order" for lack of a better term) are the good guys is being challenged, with similar revelations for the cosmic forces of Life and Light.
So, what do you think a Light-aligned but morally evil entity would look like in the Destiny universe? Personally, what comes to mind is some type of...cancer cenobite... an entity obsessed with experiencing everything there is to experience - to the extent that they will happily torture existence into experiencing something, everything, and anything at all. No matter how hard existence pleads for it to stop. Like if the fan-theories of Nezarec secretly opposing the Witness' goals were true - because finalization would mean the end of suffering and nightmares too.
I'd like to start a discussion on the recurrent themes of Destiny that we've lived through over the last 12 or so years. Whether they were placed front and center (such as "Guardians make their own fate") or more in the background (such as the concept of the Egregore showing up across different groups touched by the Darkness - including the Precursors, the Seven Seraphs, the creation of Lakshmi-2, Calus and the Glykon, hell, even the Hive joining with the worms to cast off the mortality of the Krill).
I want to leave this post open-ended to not cut off any possibility-spaces of discussion, so I'll just vaguely gesture towards what I see as some of these recurrent themes to hopefully springboard their further discussion:
many individuals giving up their individuality to join into a collective mind, in the pursuit of power or meaning
Competing Triumvirates and the mirrors they held up to ourselves - eg the Vanguard/the Hive sisters/the Dark Vanguard all containing facsimiles of the Hunter/Titan/Warlock
Fate versus indeterminability - From the very birth of the Universe as we know it, this has been one of the core themes, with The Pattern (which in our universe has taken on the shape of the Vex) vs paracausality
The dichotomy between the physical and the mental - this one is slightly more debatable what with the "changes" made to the Darkness over the years, but I think we can look back at (for example) the Dreams of the Alpha Lupi and see even back then this notion of mind/memory and physicality being at odds, before being made much more explicit in things like letters between Ikora and Sen-Aret
This is just a loose outline of the recurrent themes which first came to mind for me, I'm sure there are many others, and much more detail which could be added to the notions I've started off with here - please, add anything that comes to mind!