An optional pvp justice system in your player house. By default it's disabled but there could be a furniture item safebox that once placed will generate valuable fence items over time and thief players can break in and try to rob it. With the item in their possession, they're flagged for pvp with the player that kills them getting a bounty reward. If they reach a outlaw refuge there will be a large value when fencing the item.
This won't interfere with pve thieving since in order to participate the thief would need to actively go into a pvper's house and steal an item specifically for this purpose.