Maybe you could nerf the headshot itself, instead of an instant kill it could be an increase in damage at farther ranges, or just a remove headshots and give them a higher power? Idk just tossing around ideas
I honestly don't think Snipers or Assassins need nerfing. The only thing I think needs nerfing is the Pyro's explosive arrows. They deal 5 hearts of damage if they come 3-4 blocks away from you! They can keep the bow, just nerf the explosion.
Actually I feel for the pyro it should be made that the arrows only explode if shot a certain distance and only use a flame effect if shot from shorter distances. Because as a ninja fighting a pyro is impossible all they have to do is quick fire an arrow with know need to aim and they can light the ninja on fire. It's like a get out of jail free card.