Right now, like a lot of people have pointed out/complained about, there really isn't any mechanics that support players who want to work together. Blood bags are an exception to this. What if we had huge, enterable buildings that are absolutely filled with zombies? (Malls, Military Complexes, etc.) They would also have lots of loot, it would encourage players to work together to clean them out for it. Plus it's an opportunity for the terrifying, claustrophobic fighting against hundreds of zombies that games like Left 4 Dead are famous for. The ability to repair trains and train track would be awesome, as long as it took huge amounts of material to repair, and then later keep it functional; more than is feasible for a single player or small group to reasonably acquire. Big, hundred foot yachts could also fall in this category (Heli pad in that back maybe?) This would encourage players to actually group up and work together to accomplish something, instead of just backstabbing for the loot the other guy has.