Either a Tesla inspired Tzeentch DarkMech or Undivided stalin-esque human warlord if playing in the vortex or primarily chaos space.

If playing in the imperium either a nurgle follower in the administration who does his job helping the bureaucracy of of the imperium get more bogged down and and speed it's decay or a decadent slanneesh noble.

What about a insane warrior who has fled to the Screaming Vortex to get to the source of the toxins and booze that can only be created on a daemon world because nothing else can get him high/drunk anymore and he wants the sensation of these things far more than he wants to keep those around him alive to do it?

While I would favor more "mundane" followers of Chaos, I recently uncovered a Chaos Space Marine I think we should all try:

http://blazbaros.deviantart.com/gallery/#/d3e1vgp (yes, this is safe for work.) BREAD FOR THE BREAD GOD!!!

What about a Kroot who is part of a band of Kroot who had feasted on far too many corrupted agents of chaos that they have ingested the taint as part of their dna? The leader of this vicious warband is a fat, diseased Kroot on the scale of one Jabba the Hutt. At the start of the game, the chaos kroot go out in search of more chaos-tainted creatures and people to devour! Mmmm warp spawn!

My current concept for the campaign we have slated to begin as soon as the book comes out is a guy who barely made it onto the last boat off of some doomed world that was being devoured by the tyranids and he watched his Guardsman father and brothers getting slaughtered and eaten behind him. This left him with a bone-deep certainty that the Tyrannids are the worst threat to mankind that exists. As the orphan of an Imperial Guardsman he went to the Schola Progenium and spent his years there trying to learn how the Tyranids could be beaten. He came to the conclusion that they could not be bested by conventional means, but only on their own terms. The bio-weapons had to be fought with bio-weapons. His research into germ warfare has led him naturally to the service of Nurgle. He is certain that he is using the terrible lore of chaos to a noble end, bring about the destruction of mankinds greatest foe.

We'll see where it leads...

Oh, and he talks, of course, Like Dr. Evil.