I think a reward system would be helpful towards the game, but what would it reward?? How many zombies you killed, how many bandits died at your hand, how far you have ran? It's really difficult to see what kind of reward system you are going for because, even if it pops up with achievement you killed a zed, then what. What do you receive for wasting ammo. You can maybe get crafting recipes when they implement crafting, but that is a longshot, what would you get rewarded with ?