Being on the ground could give you a 20% chance of projectiles flat missing cause your a harder target, and some combat styles with “take cover” tags could raise it to 50 or 75%.

it could also reduce explosion damage…

Military Training- a combat style based on gun to gun combat.

To add to this idea: make prone characters and enemies invisible through “tall” terrain? For instance, windowsills, furniture, shrubs, rubble, shrubble, shallow water, and pits?
Prone should make you move slower and very poor at melee, but maybe allow you to hide in/behind bushes and furnitures, as well as let you get to and close a window without broadcasting yourself to a street full of zombies.