The old, old server I used to play (Fake Perfect World) had this. You could PK anywhere except spawn points and Ancient Dragon City (Archosaur for you kiddos). I liked it because it worked like the Arena, if you left SZ - you left, there was no going back unless you used a teleport stone or ran to ADC.
#BackWhenNpcR8WasEndgameThatDidntHaveToBeRolledOn