The Inverted Chaos Gods and their potential champions
Aright, let's start this off with an easy question: if there are four Dark Gods, and if we ignore Malice throwing a tantrum in the background because once more he was ignored and in this case even deliberatly, why does the official Chaos Symbol look like this?
That's a bit more than four going on, twice that number to be exact. So as canon lore hints at, its that the Chaos Gods themselves are not exactly stable entities. They might appear so to the perspective of a mortal human being, but they are suspectible to change even if they themselves might not exactly like it. If the cultures and species in an entire galaxy shift and change over the milenia, so will the gods who are shaped by those in said galaxy.
There is this bit of lore that Khorne was formed due to the terran middle ages, which at first sounds like a joke: how was one backwater savage planet with not even a billion hailress monkeys able to spawn a literal god? The answer to that is as simple as it is anticilmatic: they did not create Khorne. They, however, did make up a VERSION of Khorne that eventually would become a smiliar version to the one we all know today. At the same time, there was probably not just a single version of Khorne - while the humans had their version, a few systems over there was still murder and bloodshed going on by some other backward planet. They shared absolutely nothing with humans in terms of origin or culture, but the fundamental concept of violence and the institutions of civilication to control it were the same and fed the same warp-entity that was also existing on terra.
There's just one issue: the warp and reality are not exactly compatible. In reality one has to adhere to the laws of time and space, and if there is a thing that exists in two different places at the same time, they are, by definition of the universe, different things. While deep down "alien Khorne" and "human Khorne" might be the same thing, when interacting with reality they might even be considered almost entirey seperate entities with possibly no knowledge of the other. As this is basic mechanics of the divine in the setting, one could assume that this works the same for all of the Big Four.
Also, we all know that the Chaos Gods are not evil in their entirety, but that they also include their "non-evil" counterpart. Khorne for example is violence, but also the mechanisms to control said violence, like honor or "rules of battle" that forbid stuff like targetting medics on the battlefield or murdering civillians. This is where the eight-pointed-star comes from, as it is not 4 Gods, but actually 8, each of them also containing in their own mirror-image.
So if we assume that the chaos gods could have aspects of them running around that even they themselves are only aware of on a subconscious level and we know the nature of said gods as changing over insane amounts of time, wouldn't it be too weird to assume there are "variants" of them running around that are "not evil"? Possibly from before the Warp got all messed up, or maybe it just spawns occasionally before it gets absorbed by the currently stronger concepts.
Assuming those variants run around, they must be SIGNIFICANTLY weaker than their bigger evil counterparts - and the weaker a warp entity is, the less power it can use when affecting reality. It therefore requires someone to act in its stead, a "Chosen" or a "Champion" if you will.
A faction surrounding such a Champion would certainly be in a weird spot: the more they help their variant of a chaos god, since in the end it is the same entity, the stronger the "bigger version" gets too. So the end goal would ultimatively to "reform" the image of the whole thing so "their" variant becomes the dominant one. A risky play, mostly because in the end it could all be for nothing, the "good variant" gets absorbed, and in the end you got an evil god twice as strong as before.
But you know what's the best about this theory? We already might have one of these Champions running around on the tabletop!
Let's see, Khrone's favourite color is red, he likes melee combat, can't do anything psykery, his favourite number is eight, and the "good" aspects of him can be distilled to honor and chivalry.
How curious then we got this honorable red lad with no psykic powers leading his combat group called "The Eight" being the only one in his entire culture to swap a railgun for a melee weapon. The T'au are a culture that abhors melee combat as barbaric, and yet here we are getting the guy who gets up every morning and chooses melee violence as his choice of combat. Oh, and his break with the T'au Empire happened over matters of honor (they lied to him) right after encountering Khrone demons.
That's a bit too many coincidences piling up, don't ya think?
But that's just Khrone, now where are the others? We still got three gods left after all. There's two options here: either they all come from the T'au, and this is somehow an milenia spanning plan to reform the Warp, OR these Champions will appear randomly in all the factions save for maybe the Tyrannids.
When trying to make predictions on who goes where, maybe Farsight's case could be an indicator: regular old Khorne has little to zero interest in the T'au. His initial presence with them is fairly low, so if there is a variant of going around, it would fly under the radar for quite some time. With that in mind, let's assing the Champions of the Inverted Gods to the various potential factions!
Slaanesh: Being a non-evil mirror to Slaanesh is probably weird, as excessive emotion, sensation, and pleasure are not exactly evil as long as you are not catholic. But if there is a faction that is naturally immune and also irrelevant to Slaanesh just as the T'au are to Khrone, it is the Necrons. There must at least be some Overlord who wishes he could feel the wind on his skin or smell the flowers once more - and to use that to free his warriors/servants from their mindless existence. Their goal would not to have an excess of emotion and sensation, but any at all really. It would be a Necron faction that cares about art, about aesthetics, and maybe also about genuine emotional connections. You could call it the "Romanticist Necron" faction if you will, and other Necrons would probably not get along with them.
Nurgle: Nurgle has always been about nature, the cycle of life and death, and all that stuff. The thing is, with him being both life and death, he steadly keeps everything stagnating with nothing ever truly dieing and making way for whatever new thing could rise from the ashes. Now, who could qualify as a very stagnant faction that at the same time holds little interest to the big guy? The Aeldari - they do not die natural deaths, and when they die either their Soulstones or Slaanesh gets them. They might even be actively on the way to do that with this whole Ynnead project, but given the setting it might as well brutally backfire. You don't just bring a god of death into 40k and expect it to work for you, no?
Tzeentch: Being a god of magic, grand plans, and schemes, Tzeentch hates probably nothing more than the Orks, dumb brutes bashing in their heads for millions of years without anything ever coming from it - despite them actually having a lot of psykic potential with their Weirdboys and their Meks showing some insane innate understanding of warp-infused technology. But just imagine the concept of an Orc-Psyker able to rival Ahriman, while not exactly caring about Gork&Mork. Imagine, if you will, the impossible: a smart, calculating, and scheming Ork. Not in a Bloodaxes sort of way, but in how a regular Tzeench Champion would operate. A small subsection of Orks becoming actually smarter, knowing when to hold back and how to use diplomacy and politics, might be something that should give half the galaxy plenty to worry about...
