As I understand the next goal for Rocket and the development in DayZ is to increase team work between players. There's been talk about radios, and they may well be necessary to move people from TeamSpeak/Skype to in game communications, but here's somethign else: Megaphones! The main problem as i see it, is that you can only talk to other players at distances where you can also shoot them. So after you have acquired your first weapon, it's a big risk trying to chat with anyone, and most people just resort to shooting at first sight. With megaphones you could talk to other players at greater distance allowing you to communicate without being shot immediately. Sure it's still risky, since it will give away the direction where you are, and it might even attract zombies, but it's still much more safer than walking to some stranger trusting that they will want to chat with you and not just start shooting immidiately. Megaphones could also be used to coordinate bigger groups; something like this:
I guess there should be some kind of mechanic/echo sound added when speaking through megaphone, so people wouldn't assume some one is right next to them, when they hear some one speaking. Inventory wise, the best way would probably be to make it a tool: can be on tool belt, or as a secondary weapon. What do you think? What ever the solution for the team work problem will be, i think it must come by enhancing the ability to communicate in one way or another.