Towers are a kind of turret, food is for health and by orbs are bosses.
Extra suggestions:
-Working spots for npc's (farms, woodcutters hut, fishing hut, etc.)
-Shooting holes for in the wall. (after the wall bug is fixed)
-Barbed wire (for slowing zombies down)
-Animal traps (for animals if they will be added)
-Perks/skills (like faster walking, more food from trash cans, more wood from trees)
-ability to fish (like small fish, medium fis, big fish and each one gives a diff amount of food)
-Pets (like a dog or cat maybe a bird that fly around the map searching for survivors)
-Hostile survivors (not everyone is good.. DEAL WITH IT!)
-Crafting items (like nails that you need to find or a hammer.. maybe parts for a special chainsaw)
-Diff cars (like a truck whit a gun on the back rly fun when playing with friends)
-Raids (diff city's that are attacking)
-A option that teleport all the players when player one enter a diff area (because some people are just faster then others)
-Diff kind of zombies (Like militaraire zombies that give bullets)
-More traps (its a defending game with 1 trap... maybe some traps that you can re arm like bear traps)
^^here a hole list of ideas,
thx for spending time on reading it :D!