The exact mechanics of how this would work would need some thinking, but I think it would be really interesting if you could sneak up on an enemy, then your sword attack would be an instant kill.
Perhaps you are in "stealth" if you haven't been in the enemy's field of vision for the past 60 seconds or something like that. And it would be up to other players to turn around and look at you if they hear you walking, to avoid being stealth attacked. You might need some sort of indicator to know when another player is "aware" of you.
It would be an incentive to play a bit more sneakily, and perhaps even make tuck scenarios more interesting.
