SuggestionsDecorations/Props- Add more debris to the surroundings and in houses, such as paper, maybe dead body's, and blood.
Items/Weapons- Machete, Bat, Bat With Nail, basically every day items. Maybe even the M9, 20 gage (shotgun) ect.
Environment- Add more of a dynamic environment like fog when it rains, temperature that if you get too hot/cold that it will slowly kill you and add armors/clothing so that you can survive what every biome your in.
Mobs/Zones- I know that some zones have been added but you could add mutated walkers (zombies) run super quick or some be very slow and beefy looking because either they're fat or they're so decayed/covered in clothing or rotting flesh they walk slower but deal a whole lot more damage. Would look good for hospitals or heavily over ran areas.
Talk mod- Lets players communicate over the mic without discord making it more enjoyable to interact with others instead of typing and possibly being killed while doing so.
Looting Chests- To free up the default chests for storage uses and other reasons, making a block that is lootable and has a similar loot to the chest but only a little different to make it stand out and make some locked so you need a lockpick to access the chest to get the loot. Maybe even make it so some items can break into the chests?
I am aware that some listed suggestions may not be stated clear so ask anything in the comments below and i'll answer. Hope some of these make it in or at least something similar to what I have added.