Just go back to the basics. The mansion. Make it small, small group of people (STARS team) cut out of the world on the mission, claustrophobic feel of tight halls and make ordinary zombies scary again. With bigger picture going on in the shadows, finding logs and information as you explore mansion. As same group of STARS trying to survive with limited ammo in creepy mansion and trying figure it out what is wrong with it, giving many doors are locked and work on principe of the puzzles. Only to realize through exploring and surviving it's actual secret umbrella facility for BOW.
As someone from the group is traitor and purposely trying to keep them in mansion, and cover it up. I just desribed first Resident Evil game. Do it, damn it.