I've been having some thoughts. Let me share them with you!

For as long as I can remember, the four great Chaos gods have always been seen as equals, and this has always supposed to mean equals on the battlefield. Which we the customers have always wanted to mean equal in terms of models support. With so much bias towards Khorne and Nurgle recently, and recent rumours over Slaanesh disappearing, it has made me think...

Perhaps they shouldn't be so equal and symmetrical? Consider this... Khorne is the god of bloodshed, war and battles... Doesn't it make sense that we saw more of his followers on the battlefield, as that is all they live for? Slaanesh and Tzeentch are more subtle gods, doesn't it make sense that their followers are less of the hulking barbarian type, and more thus seen less on the battlefield?

I suggest a re-imagining of the four Chaos gods could be interesting, and possibly this is what GW have planned, though I do doubt it.... So this is what I would do if I was in charge!!!!

The four chaos gods should represent temptations that mortals have given in to.
Khorne = violence
Slaanesh = pleasure
Nurgle = resilience / life / death / immortality /etc
Tzeentch = knowledge / sorcerous power

Khorne should represent the armoured hulking warriors doing all the gritty business of killing and slaughtering. So his followers are the easiest to represent on the battlefield. I propose all the current fluff we have for marauders and warriors of chaos apply to Khorne worshippers exclusively. This is kind of what seems to be happening with Age of Sigmar (perhaps). All the soldiers, mercenaries, knights and other mortal warriors who have got a bit too much of a taste for violence, and have renounced all other things in favour of undending war and killing... They have developed a real bloodlust, and have gone a bit too far down that route. At some point they are tempted by Khorne to join him in a life on perpetual bloodshed, and they accept. This is where the mortal armies of Khorne come from. Those who distinguish themselves are gifted with daemonic weapons, armour, and mounts by the big K. So what we currently know as 'chaos marauders' are the masses of blood-crazed psychopaths who have thrown their lot in with Khorne. What we know as 'chaos warriors' and 'chaos knights' are those whom he has noticed and favoured. Thus 'chaos armour', and daemonic mounts (ie Juggernauts of Khorne) for the extra special ones. When these armies go to war, they are joined by the Daemons of Khorne (packs of flesh hounds, hordes of bloodletters, and the dreaded Bloodthirsters if it's gonna be a particularly big and messy fight!). Mortal followers who do particularly well can hope to one day be elevated to daemonhood (ie Daemon Princes). I suggest lots of the things we associate with Chaos in general, ie chaos armour, muscle-bound barbarians, the champion's path, and all things spiky, be moved to Khorne exclusively.

Nurgle.... I really like the recent Blightking models, and all the good conversions of super-disgusting Nurgle chosen warriors we no doubt have all seen. Here is my proposal. Rather than the old "barbarian northerners who worship chaos and fight their way to becoming champions, but with a Nurgle flavour" thing that we have seen before, it could be made more interesting. These 'blightkings', or rather all the big nasty mortal warriors of nurgle, could represent fallen heroes who have given their souls to nurgle to save themselves from disease. Thus they could represent fallen knights, nobility and soldiers from the Empire, Bretonnia, and other human nations. Example.... A heroic Bretonnian knightly hero. He dreams of a heroic death in battle. However he and all his followers are sticken with a hideous plague, and he is cursed to die a slow and ugly death in bed rather than on the battlefield - not what he wanted. Along comes Nurgle and says "give your soul to me, and you will survive, live on to fight another day, and see many more glorious battlefields." If he is is foolish, he accepts. Thus the mortal champions of nurgle - once great heroes and warriors, they have turned to Nurgle to protect them from death by disease. They live on to fight another day, but it ain't pretty.... It would be cool if we then had some more models like the blightkings that featured little references to their former glories... tattered heraldry, recognisable Bretonnian and Empire armour / helmets, etc.... These guys don't have 'chaos armour', that's just a Khorne thing now, but they are super-resilient and hard to kill (conveniently, the blightkings and lots of other nurgle models seem to show more diseased flesh than armour). Instead of 'marauders' or lesser followers, we have Plauge Zombies instead. Quite how zombie-like and undead they are I'm not sure, but they are just hordes of normal people / peasants who are stricken with terrifiying diseases, and Nurgle manipulates them and uses them as cannon-fodder. And Nurgle's armies are always joined by his daemonic followers.... plaguebearers, nurglings, and the Great Unclean Ones.
The mortal followers of Nurgle are thus seen more as slaves to his will, who perhaps inside curse the day they took him up on his offer and just want to die quickly and painlessly (aint gonna happen!).

Tzeentch..... The focus here should be more on wizards. The temptations Tzeentch offers to magic users is knowledge and unimaginable power. Example.... A wizard of one of the colleges of magic in the empire, doing his research, is tempted to look into the darker side of things. Along comes Tzeentch and tempts him. He goes for it, and the rest is history. There's less room for large armies and armoured warriors here, but perhaps more daemons to compensate. An army of Tzeentch is focused around one mighty sorcerer, who could have several lesser accolytes. He is able to summon lots of daemons (horrors, flamers and screamers), and if he's extra good a Lord of Change can come and teach him a few tricks. For mortal followers, perhaps hordes of mutants? He could give Tzeentch's blessing to a whole town, resulting in a horde of bizarre and colourful mutants he can bend to his will. No real 'chaos warriors' here as we know them, as Tzeentch's followers don't live by the sword, but he could perhaps have some mindless armoured bodyguard types, either warriors with their minds wiped or animated suits of armour....
So an army of Tzeentch consists of one super powerful sorcerer (plus acloytes), hordes of writhing mutants, and lots of summoned daemons.

And finally Slaanesh. The temptation aspect here is obvious. Needless to say, groups of pleasure-seekers sooner or later take things a bit too far and are tempted by Slaanesh. The foolish ones start to worship him. The emphasis here could be more on cults of mortals, with lots of daemon buddies. It never really made sense to me why brutal northerners and chaos warriors who probably can't even take their armour off would follow such a decadent diety..... The true followers of Slaanesh should be normal people from the cities and villages of the Old World. How this relates to the battlefield is a bit trickier, as the cults would have no real military strengths. Perhaps a greater focus on daemons? Perhaps a greater daemon only turns up if the cults are engaging in particularly perverse and depraved pleasures.... Obviously not one for kids, but he/she shouldn't be. An army of Slaanesh could be a Keeper Of Secrets followed by lots of daemonette retainers and messed up human cultists. No armoured warriors here.

So what happens when a city falls to the followers of one of the four gods? Before, it's always been "armies of hulking armoured warriors descend on them and slaughter them... and they are either painted red, green, blue or pink". Instead each god's methods should very different....
Khorne - Hordes of blood-crazed psychopaths. They flatten the city and slaughter every one inside.
Nurgle - The city is poisoned and cursed by horrible diseases, and everyone within dies a slow and nasty death. Those who pray for deliverance find themselves selling their souls to Nurgle. They survive but join his armies.
Tzeentch - A wizard flying on a disc comes and blasts the city to pieces, whilst all its inhabitants (and the buildings themselves) mutate like crazy!
Slaanesh - Cults of pleasure rise up inside the city walls and things get kinda anarchic. Daemons appear and it turns into one messed up orgy.... Not for kids.

So there we have it. Perhaps the four gods shouldn't be so equal and symmetrical in their representation on the tabletop?