I think it would be neat if Owlcat adapted Only War, since they're doing Dark Heresy now.
You could have your character be an NCO officer like a Sergeant or Corporal from a regiment of your choice, and you are deployed to a warzone against Tau. First act of the game is mostly fighting Tau, meeting characters, and basically being introduced to the Imperial war machine, and your part in it. Then Chaos attacks, forcing you and survivors from other regiments banding together into a rag tag company of heroes that fight to survive and hopefully find some way to call for help or get the hell off world.
Third act you would have teamed up with Tau(or exterminated them) to face down the heretics before they can summon a daemon prince.
Movies like Where Eagles Dare, Dirty Dozen, Saving Private Ryan, Kelly's Heroes, and Guns of Navarro would be a big inspiration for tone and set pieces.