I've been playing 7D2D for a year now, and i would like to suggest new NPC Mechanic, new Vehicles, vehicle mods and Turrets
For NPC Mechanics
1. the trader could give you quest to get him some security that defends the trader and Zombies could destroy it so that there are no safe areas other than your base.
2. Bandits, have Bandits or raiders roaming around and have a Reputation system for each bandit group. For example let's call the first bandit group (Arman) so if you attack (Arman) you loose Reputation with them. When you get to a certain rep with them by attacking them, they would attack you on sight and your base if they are within a certain range and raid you, If you attack one of their enemies on the other hand you would gain Rep with them and you can trade with them and help them out or hire them to go on an expedition to gather stuff for you, but you would also loose rep if you don't help them get rid of hordes or attacks from other groups
3. Military NPC'S that are in the military bases and they will attack players or bandit groups IF they are walk around with weapons in hand (Spear's, stun baton, Rifles, SMG'S, Hand guns and so on) but they don't attack you if you walk with tools in hand, you would also be able to get quest from them (Save one of their soldiers) and these quest would be End game quests and in return you could choose either a weapon or ammo. While also being able to buy from but at a more expensive cost
1. A boat that you can craft that uses a sail to move
2. A Humvee where you can put a turret on and it have room for 5 players 2 in front, 3 in the back seat or 2 in the back seat and one on the turret. but the turret will get destroyed the more you use it, and you have to have the ammo for it in your inventory.
3. Being able to shoot from the vehicles with rifles or hand guns in the 4x4
4. Add destroyed tanks with ammo in it that player can scavenge. and get parts of
5. Make room for more players in the 4x4
6. Transport helicopter for the military (Can't be used by a player) that will fight back if shot at and contain tier 4-6 weapon loot or food/water
1. Turret mod for 4x4 or Humvee that a player can use that uses special ammo like 50. CAL ammo or other heavy ammo can be crafted through a recipe or bought from traders or military NPC's
2. Spikes on wheels mod that shreds Zombies to pieces if they get hit by them. this mod can't be crafted only bought or found and it should be rare to find
1. gattling turret to shoot down military helicopters
2. being able to use other ammo for SMG and Shotgun turrets
All of this is just to make 7D2D harder and so that the player have to watch out for other stuff other than zombies and animals and give it more life