Brainstorming idea: WH Chaos cults that embrace aspects of chaos that are often overlooked. Because half of my players are metagaming bastards and I don't want all the mysteries to be solved like "oh yeah these guys are x."
Nurgle: The Unbothered
A chaos faction that embraces nurgle's aspects of decay and entropy, instead of his usual rot and disease. Their domains are ancient ruins guarded by warriors who don armour whose paint long since faded away; where they walk, machines malfunction, weather patterns stagnate, fuel cells deteriorate and people become lethargic and despondent. Their ultimate goal is to achieve the heat death of realspace and the triumphant suicide of chaos.
I like to imagine these guys all have very refined and aristocratic accents befitting a long lost era, and have inherited Nurgle's stoic attitude towards anything occurring. They sway new members into joining, promising that although they cannot make their problems away, they can take their worries away. As a result this nurgle faction is often allowed to proliferate on menial factory worlds and agri-worlds, with Imperial overseers often pleased that dissent is at an all time low, productivity is at an all time high, blissfully unaware that the unbothered are subverting the world with calm and purposeful intent. When they aren't planning some grander machinations, they enjoy wasting non-renewable resources on idle trivialities, often encouraging the more excessive habits of Imperial aristocracy. As such they have a mutually beneficial relationship with a Slaaneshi faction, known as Dissolute Merovingia.
Apostate ΔP (Delta P)
Apostate ΔP was a low-ranking Enginseer who would have lived a dutiful and profitable life, had she not chanced upon a Horus-Heresy era incarnation wheel. Unlike her peers, ΔP was cursed with original thought, curiosity and the will to innovate. Instead of reporting her findings to her superiors, she began first to investigate the machine's functions. She had no problem at all navigating the user interface of the 10,000 year old device, the ease of which would have triggered the appropriate paranoia of any inquisitor - but seemed to ΔP proof that she was being limited by the dogma of the cult mechanicus. Her experiments began small; reviving first a rat, then a dog, then a grox. Some times she would revive creatures partially, or assemble chimerae, or experiment on servitors - things in between life and death, advancing her knowledge piece by piece. By the time she moved onto human subjects, she was declared an apostate and tech-heretic of the cult mechanicus, spending the rest of her life on the run from the mechanicus, the dark mechanicus and the ordo malleus. Apostate ΔP would have perished long ago had she not found a profitable alliance with a radical Inquisitor of the revivifactor faction within the ordo malleus, who was keenly interested in her work on bringing human souls back from the warp. It is unclear who amongst the hunters and the hunted is aware that the incarnation wheel was a device made in worship to nurgle, lord of rebirth and despair. For the incarnation wheel can bring souls back, but its cost is despair - the more potent the despair, the more power the machine collects.
Tzeentch: Mask of the blood god
A chaos cult that wears red, screams Khornate slogans and fights in melee. Actually a cabal of sorcerers doing their best to piss off Khorne and trigger retribution against a rival Tzeentch cult, the thin men.
Their plan is going to backfire ridiculously when Khorne approves of these sorcerers eschewing magic in favour of melee, and they're going to accidentally become an actual Khornate warband who uses their knowledge of sorcery to teach their followers how to hunt psykers and sorcerers. Just as planned?
The thin men
The thin men are nine xenos masquerading as humans who have forseen their deaths, and are manipulating events to ensure that their destiny is guaranteed. The thin men worship the god of fate and destiny with zealous revery, and believe everyone should march towards their destiny with confidence - after all, everything is in accordance with the architect's will. Whereas most tzeentch cults are subtle, the thin men are on a speedrun any% to piss off as many factions as possible to assure their own swift demise. The only catch is, they have to all fall together in the exact way prophesied, else they have failed to follow Tzeentch's will.
Khorne:
Layman Octan
Octan started his career as a mere scribe-adept seconded by the administratum to work under the planetary governor. Possessed of a glib tongue, sharp mind and iron will, Octan steadily accrued influence far in excess of his station as more and more officials depended on his counsel and more and more of his opponents met quick deaths. In his jurisdiction, when a judge acquits a murderer on a legal technicality, Octan's influence in the ruling is seen but not heard. Octan is a devotee of Khorne, as such he strives to ensure that honest murderers who commit crimes of passion are excused, or given more opportunities for violence, resulting in Octan accruing a steady backlog of killers who owe him favours. Not all convicts and accused are worthy of Khorne - Octan strives to ensure cowards and killers who kill for pleasure like a disgusting slaaneshi never see the light of day. As a result, to all outwards appearance Octan seems like a fair and impartial man.
Kalamat Pact
Besieged and isolated, many an Imperial world has died in the void without the Imperium noticing. Sometimes such a beleaguered planet turns to anyone who can help - and sometimes their calls are answered. Brass ships in the void appear and disgorge legions of disciplined warriors from the Kalamat Pact, who hail from a small human Empire that has existed outside the reach of the astronomicon and the imperium since the dark age of technology. Like most Khornates, they favour direct assaults on enemy positions. The Kalamat Pact however employ heavy use of combat engineers, close air support & artillery to ensure their success, using strategy and discipline often neglected by their more enthusiastic peers.
Slaanesh:
Dissolute Merovingia
The DM have their initials everywhere; they are sponsors of the most excellent art works, plays and holo-films, and their works are sufficiently patriotic that they make it past Imperial censors mostly intact. Skilled critics however can correctly interpret the subtle satire and undertones which quietly point out the Imperium's all too human flaws... But besides a few eccentricities, most artists sponsored by the Dissolute Merovingia would never be able to tell their patron is a front for a Slaaneshi plot. Except perhaps when their actors are forced to redo a scene 211 times because the scene is never perfect enough.
2GOD
After discovering the secret of Slaanesh's genesis, the ordo xenos inquisitor Hayak Busano went rogue, being declared traitor excommunicato in absentia. Hayak believes chaos can ultimately be bent to humanity's will, and believes if one god could be made, then another could too. The rest of the inquisition is not keen to find out whether it is possible to make a second Slaanesh, and so it is only a matter of time before Hayak is hunted down and killed. Nevertheless, he leaves a trail of research just waiting to fall in the hands of those around him, ever widening the net of suspects who must be eliminated