Then...Well as you said Tzeentch. Being random is cool.
Then I'd try Nurgle. A nurglist doctor. Doctor Frank Einstein, I presume
Slanesh and Khorne feel a bit too "obvious" to me. Though they're cool, too.
I don't think any of the Chaos gods is "obvious" if you don't insist on making them so... I'll give a few examples:
Imperial Cleric who wanted to be perfect in his worship of the Imperial Creed... Except perfection is pretty damn hard to achieve, especially in faith as multi-faceted as Imperial Creed is. But hey, don't worry, we in Slaanesh cult know ALL about perfection... We can make it happen
Adept of Administratum who is very competitive, always trying to be the best in his office, winning everyone else in speed and accuracy of his work. When he can't quite seem to get above his peers he asks Slaanesh for help to perfect his working methods... or, starts to ask hep from Khorne in methodically wiping out the competition.
Oh there are so many wonderfull ways to be corrupted... No need to go with the stereotypical "Khorne Psychopath Pirate", "Slaaneshi Drug-Addicted Nymphomaniac", "Tzeetch Meddling Sorcerer" and "Nurgle Deathguard Zombie" concepts...