I just found out about onslaught mode. Why is it hidden? In the tutorial a zombie spawned before it should. Sometimes zombies just disappear randomly. The tutorial guy just walks straight through the gate without it opening. When start quests, if later in the quest you need to collect items (ex: donut,wood) but you are able to get the quest versions where it spins around your head before you are told to get it.
Make a health bar on structures so you now how long until it breaks. Make it clear you can skip the opening intro. Make it so on hard and onslaught npc that you were not able to save because the died stay dead and don't respawn when you load again. Make it that most or at least some cars don't have fuel and are already damaged like missing doors. Make the metal on cars not respawn when you load again at least on hard/onslaught.