When building a campaign, it's important to remember that Chaos is, by its very nature, diverse. Slaanesh isn't always about having sex with things, and Khorne is not just a mindless rage monster. The campaign I'm running right now has a party dropped in a system after they escaped a prison ship, and they got invovled with local politics and tried to carve out a niche for themselves around the worlds. They also chose to defend the system when a Tyranid Hive Fleet came to invade because they felt invested in the system there.
Here's some examples of Chaos worshippers who would be considered atypical, but still in line with the Chaos Gods:
For Khorne, you could have a noble warrior who favours martial skill above all, and will only subjugate or tax the defenceless until such time as they put up a fight. (Such an example exists in the fluff in the BC rulebook)
For Nurgle, you could have a mad doctor who keeps a pristine laboratory, except for the areas where he tests his new vile toxins and contagions upon his helpless subjects.
For Slaanesh, you could have a craftsman or warrior who absolutely devotes themselves to their art, always seeking perfection and seeing their past glories as old and dull in favour of the latest high.
For Tzeentch, you could have a naive rebel, not a schemer, but a pawn in the schemes of the Ruinous Powers, who believes that as long as he's devoted, everything going just as planned will work out for the best.
And while those aren't automatically what you think of when you think of devoted worshippers of those Powers, they're still perfectly viable.
I quite agree with this sentiment. The extreme stereotypes of the chaos aligned are just that, extreme examples. Personally I encourage my players to break those molds. In one of my games there is a nurgle aligned apostate who is helping build the economic strength in the sector we are playing in. He is doing this because by building that strength he is also morally corrupting the governing bodies in the area. Not corrupting them to chaos, just morally turning them from good to expedience, then from that to full on greed. When he is done with the sector, its civilization will be in decay, and he will dedicate that decay to his god. Plagues and poxes will have nothing to do with it, but nurgle is the god of decay so it is appropriate. This is the kind of non-stereotypical character I like to see.
I have another player playing a thousand sons sorceror that has come to the conclusion that the ultimate homage to tzeentch and insult to the imperium would be for him to be responsible for succeeding at reviving the emperor himself. To that end he has abandoned his primarch and legion and formed an alliance with one of the necron dynasties, promising to help them raise the martian tomb from its slumber. While I dont think he will manage any of that, the player has told me he really just wants to make an epic, and epically annoying, recurring villain for my other games.
But please for the sake of all that is unholy, leave the generic characters to the npcs where they belong.
I also agree with others that BC is better play by letting the players form thier own plot. Pf course I think that is better in most games, but I think it also takes players who will do more that sit along for the ride. For the most part I personally despise GMs that try to railroad things. DW and OW are better for the "this is the plot, deal with it" type of GMing in my opinion.