I think it could be fun to add the ability to place a bounty on another player's head. I imagine it being like when you hide treasures and share the maps:
You can place a bounty on someone's head or on a crew, using your own golden coins (if you choose to place a 2000 gold coins bounty on someone, that money gets drawn from your savings and placed on the contract)
That contract could be accepted by someone or a crew at an outpost (and would then become invisible for other players, so only one crew can get it.), at which point they would always see the prey's ship on the map. Meanwhile, the prey will get a one time warning that they are being hunted, but not by whom. (that warning would of course reappear every time that person connects to the game).
If the bounty hunter sinks the target's ship and kills him, he gets the gold coins, but if the target manages to survive, kill the hunter and sink their ship first, they get half the bounty money, and the contract closes (the other half is given back to the person who put a bounty out there, with a message that the player got away).
I think it would be an interesting mechanic to add to the game, because it would allow a player to settle grudges or have a fun hunt with other players, while having other people give it a go and get some money out of it - if they think the money is worth the danger.
From a lore point of view, pirates would definitely put bounties on enemies out there if there is a grudge to settle.
I could think more about balance and all, but I want to put the idea out there first, to see what you all think about it ;)