I have some options / methods about how you could implement open PVP systems in the game:
Have the ability to switch on PVP at any time. When this happens your name turns red and you can attack any other player with PVP enabled. You cannot turn off PVP if your health is not full (or you cannot turn off PVP for “x” amount of time).
Have the ability to switch on PVP at any time outside of the starting zone. You can then attack pvp or non-pvp players within “x” amount of levels of your level (e.g. if x = 3 a lvl 15 cannot attack a lvl 11, but can attack a lvl 12-18). Then in certain danger areas you can attack players irrespective of their level. However once you switch on PVP, all players can attack you without having to switch PVP on themselves.
Have certain danger areas that are completely open PVP. Players are warned before entering.
When a player dies in open PVP, they should drop the most valuable stack in their inventory and a fraction of their gold.
When a player gets a kill in open PVP, they should recieve a substantial amount of XP depending on their vicim’s level. However they should also get a bounty put on their head; and the more kills they make, the higher the bounty becomes. When someone kills them they get to claim this bounty.