Make it more scary, and instead of animals, use vampires, ghosts, witches, necromorphs, the "thing" infections, werewolves, grandmas climbing on walls into the mix.
Introduce vehicles to move around the map like a M1A Abrams, Stryker, LAV, Apache or Cobra helicopters that the players can use for a limited time. These vehicles come with all the standard weaponry that can be used. It would be awesome to be so powerful you can blow everything in your path while feeling so safe and secure. To use the vehicles, you have to fork out (very expensive) money like fuel for the vehicles.