Here are some various ideas I have had, they may or may not be a good idea, just brainstorming.
Pirate Island-
An Island that is a large town or settlement, kind of a public server hangout where a lot of people could be at once, 100? 200? Maybe there are NPC’s hanging out too. You lose all loot and emmisary status when you go there to keep people from running to there, but you keep supplies. It is a safe zone for the most part. You can challenge people to a duel and fight them in an arena. Duels can be watched by other pirates and bets can be placed.
Gambling hall- A place where you can go on pirate island and play card games, checkers and various other table games with anyone who is on the server.
Other ideas- archery range with contests, bowling alley, unique stores and shops.
Pub- a shady pub by the docks, maybe there are mysterious NPC’s with bounties or unique missions.