Ralackk: They could take a page from call of duty(I know hear me out) and have branching character story lines. So you could have your suvival horror fps as the puny PDF trooper, a Callidus assassin and a Space Marine. They could intertwine the storys and switch your role throughout the game so you get to see a battle against chaos from all different perspectives.
Perhaps the PDF would take place earlier on in the story and by time the space marines show up the planet has already fallen to chaos.
klaymen: Or maybe the PDF trooper can die when the planet falls to Chaos and daemon invasion (which would be quite expected from a puny trooper). Then we could play as a Grey Knight or Exorcist Space Marine, instead of Smurfs or Blud Rehvens.
Oh, and lest I forget, less "blood 4 teh blood god" Chaos and more other gods' representation. For example Slaanesh could use some shine in the spotlight. On the other hand the devs should be more careful with Slaanesh, unless they want to attact the "ZOMFG a nipple!!!1 Ban the game nao!!" people.
Sort of like Aliens vs Predator. PDF would be horrifying, you could play as a deamon and hope around on ceilings and stuff and tear apart your enemies, and then you could be a Grey Knight and feel like an unstoppable killing machine with all your Force Weapons and jazz.