klaymen: FPS as a Guardsman or PDF?
Well, a Stormtrooper may be viable as a playing character, since they are the best. But playing a Guardsman who dies after enemy sneezes in his general direction?
In my mind it's the best way to capture the feel of the 40K universe. Joe Blow wasn't good enough to be drafted into the regular guardsmen, so he's a trooper in the poorly supplied PDF of some back water colony. It's a ho hum dull life.
Then a Chaos cult rises up, and the populace starts to succumb to the taint of Chaos. Whole divisions of the PDF forces turn traitor. There's a small chance the Imperium will some regiments of Guard to assist, but don't expect much help beyond that, no Space Marine help for some world which doesn't produce anything important. You're all alone, and you slowly start to lose every ally you have to death or Chaos. Finally, you are driven mad by constant exposure to the horrors of the Warp. The game end with you either going mad and committing suicide, or embracing the madness.
If done right, I bet it could be a really fun survival horror FPS.
What makes it so compelling, to me at least, is that's what would probably happen to somebody like you or me if we lived in the 40K universe. It makes you feel small and insignificant in the grand scheme of things.