Ah, the Keeper of Secrets! I've been longing to share my stupid ideas regarding them. I do have a FW KoS waiting for me to assemble and paint her...but first I have to finish all my other models. Also we all know what a gorgeous model that is so let's move on.
I'm currently working on scratch-sculpting a Keeper of Secrets, made mostly from putty and the occasional plastic part (with hands and face modified from a bootleg Sailor Mars figure of all things!). Obviously though, scratchbuilding a model is a lot of work so let's focus on models you could easily convert.
The Ghorgon: This was the one that leapt to mind to begin with. It would be perfect for a more "classic" KoS styled after the old RT model in all its bovine glory. I'd be tempted to remove some of the skulls though. Other conversions could include replacing the "blade stumps" on the upper arms with bigger, more crustacean claws, sculpting some, ahem, udders on the torso and perhaps doing away with the loincloth in favour of some hideous undergarment...actually let's stop there before this goes places it shouldn't.
The Vermin Lord: Rats have long represented corruption, so this would be oddly fitting. Plus, with some conversions, it could be made to resemble the Fiends fairly easily. The weapons are quite Slaaneshi too!
The Giant: With hooves, a swollen stomach and a rather pronounced, um, "Implement of Slaanesh" this model would be ripe for converting into a KoS, specifically one representing gluttony. Plus it's very easy to modify. It is a little on the small size though.