MM, since you said that you are brainstorming a19. Consider this.
Most people arent in the forest or away from cities. so them being rather empty is understandable, but cities and town, that's where most people are and where'd we'd expect them to have died. it shouldnt feel so empty. I waited, did as you asked and played to see the feel. and it still feels empty. Can you beef up city and town areas? tie it into max alive, so many zeds in a town or city, kill them until the number say 100 for a small town, 300 for a city. reaches zero, have em respawn every couple of days. make the city feel like a threat again.