How bout a radar weapon: Same size as sniper with only ten damage, but every shot directed in a general area around the hit player is directed toward them if missed. Useful for Crazyballs and rapid fire, but lowers damage of Snipers.
@mattihew: I agree with your idea and the whole 'achievements = tank coins' thing, but you've also got to consider hackers and other unsavory types who would just put it as they've done the achievements and get the coins freer than what they already were. This way, kChamp would lose a lot of money and a lot of players, because nobody loves a hack, right? So in order to actually go through with that he'd have to have a really good security system somehow.
How bout after level 40 you keep leveling up, but instead of maps you get backgrounds and instead of weapons you get the ability to combine weapons, ex. a roller-digger or a grenade-fountain, starting out with weaker weapons until you rank hugher up. Since there's a lot of possibilities, there would be a really high maximum level other than 40. This would encourage players to stick with the game.
Holy crap what's with all the 'zombie mode' requests.....think about it, if there were single player zombie modes, then that's what everyone would be doing instead of actually battling each other. It'd be way too easy to level up, which would defeat the purpose; people would be quitting the game cause it got too easy. I mean crap, if it wasn't for COD no one would even care about zombies.
 "Run 2"){kind=icon}
 "Mud and Blood 2"){kind=icon}
 "Amorphous+"){kind=icon}
 "Ultimate Assassin 3"){kind=icon}
 "Starfighter: Disputed Galaxy"){kind=icon}
 "ShellShock Live"){kind=icon}
 "Get Home"){kind=icon}
 "4 Differences"){kind=icon}
 "Draw-Play 3"){kind=icon}
 "TDP4 Team Battle"){kind=icon}