The “zombie” factor should be a serious problem! There are too few zombies around the map. I’d like to see many more infected … I often go big cities, as Berenzino, and always happen to see in the distance only a few infected and nothing more. But tell you more … not only in the cities but also in open areas, in wooded zones and in every corner of the damn map, as it really should be!
I like to know that it is possible encounter zombies even while they are away from the city or I camped in a forest or in the mountains.
Another idea to increase fun and realism derived from zombies factor could be the introduction of “zombie herds”, think of a group of dozens of infected might appear randomly and at any place of the map.