I think Warlords and Yonko could be a good addition, especially for large servers
Warlords could maybe be assigned through the command of an Admiral, and some special properties could apply to them such as non-hostility with marines and immunity from bounty hunters.
Yonko could maybe be assigned to players with high enough bounties or Doriki, and could give them the ability to fly their jolly roger in a chunk of land as "territory."