Bounty System (PVP)
Create a Bounty System.
It costs X amount of gold (plus a fee [25%]) and a bounty is created for a player. The person who kills the player receives the bounty.
A person with a bounty on them, has a special text or title in their name which clearly states a bounty is on them.
Have a UI in game (or out of game) that lists current player bounties and their rewards.
The bounty should have an expiration date. If a player is not killed before their bounty expires, the initial bounty amount is returned (minus the initial fee, roughly 25%).
Could be fun, you would want there to be an expensive cost to stop people from spamming it though e.g. min 250k gold