I'd like to see a world event that has all 6 crews needing to pull together to tackle some giant challenge, much in the way the pirate lords all came together in Pirates of the Caribbean 3 to accomplish a common goal.
It could be something as simple as a massive skeleton fort, or maybe a giant kraken that needs to have its tentacles destroyed at different locations on the map before its head will emerge in the center and can be attacked, something that requires some planning.
I see the rewards being mostly locked in the moment all participating crews complete the event (commendations, achievements, maybe some ancient coins, even), but there could then be some loot that gets fought over by everyone who has gathered at that point, or maybe not, making it the one event where the server would work together toward a common goal and then part ways and resume their pirating after.
