Slower player speed, reduced accuracy/visibility, some sort of benefit when inside of a vehicle with the helmet on.
Bolded is a must.
Slower, meh, requires a new playertype. Accuracy won't work with everything since it would require a gun to check if the player has the hat/whatever on. Visibility is easy, and what about 500% more everything when in a vehicle? If not in one you get a 1% chance of the person hurting you to drop dead :)
Or 20% of the damage you inflict on other players will be added to your own health. The plague doctor mask comes to mind. And some sort of cloak that returns 20% damage to the attacker.
Poll