I think making future maps with it being more glorified such as more blood on objects, depressing such as darker colors. Maybe some terrifying decorations around the map if possible. Then making the guns sound badass. What I have heard on YouTube they do but the sniper. So maybe make the sniper sound like a god if possible😊
Gotta keep the maps realist. Can't go over the top other wise it ruins any sort of immersion; I think you will get your bloody/depressing/darker/terrifying stuff from Zombies to be honest. If you haven't seen the leaked/real trailer for Zombies I suggest you go watch them both.