Historically, virtually all piracy was in the Mediterranean, but in fiction the setting is usually the Caribbean and the New World.
I think everyone agrees the New World speaks more to our imagination, but if there's going to be another expansion to the map, one inspired by the Mediterranean region would make sense.
(Fun fact: There were ecclesiastical military orders exclusively devoted to combating pirates and freeing the slaves captured by them. There are hundreds of fortified churches along the French, Spanish, and Italian coasts because for centuries pirates acquired slaves by raiding coastal towns. The orders had pretty much eradicated piracy in the Mediterranean shortly before they became irrelevant after Malta was surrendered to Napoleon.)
From that I got the idea of aggressive human NPC ships patrolling the seas for pirates. It would be a change from the usually passive skeleton ships.
More likely additions I'd love to see, mentioned by FaceYourDemon, are more diverse flora and fauna (glowing fireflies, ants and other insects crawling rocks and trees, bio-luminescent algae lighting up coastal waves, jllyfsh lighting up deeper waters), and I like his idea of whirlpools (surprised they're not already in the game, now that hear of it).
(Don't know why I had to censor jllyfsh, it's a restricted word, and I feel like I probably don't want to know.)
Maybe also some interaction with nature. How immersive it would be if, for example, you fire a shot and you hear leaves rustling and see birds flying up.
Also, I'd like skeletons to spawn far less often (or by certain player activities, like digging up treasure does), because when they are certain to spawn, based on a timer, it removes the feeling of surprise or the unknown. That feeling is certainly part of pirate fiction.
When I first started playing the game, I so much wanted to explore the islands, enjoy my surroundings, watch the sunset from the beach, listen to the ambience, hoping to hear some unknown threat or curiosity in the distance, creeping closer, but every minute a bunch of skeletons spawned right next to me, breaking my immersion.
As for exploration, it would be cool if treasure can randomly be dug up by visual clues in your surroundings, and not necessarily via clues in the form of parchments.
But I accept that some of the above is either not in the vision of the game or a problem for performance.