The appeal of One Piece's many different arcs stems from how varied and different they are in setting, aesthetic, and tone. Eiichiro Oda's willingness to change up even the genre of One Piece storylines is partially why the series has remained in the mainstream for so long. From Thriller Bark, a haunted island of monsters, zombies, and ghosts, readers go into Sabaody, a cheerful pleasure island hiding a dark secret, and then Amazon Lily, an isolated jungle island populated exclusively by women.
So what are some other possible aesthetics that could be done for future islands in One Piece?
One Piece Hasn't Embraced the Wild West - Yet
Remarkably, One Piece has yet to a proper Wild West-style arc. An story where our lovable Straw Hats find themselves in a gut-toting wasteland island with tumbleweeds, cowboys and maybe a train robbery practically writes itself. Despite this setting not appearing just yet, there is some credence to the possibility that appears outside the series.
In 1992, when Eiichiro Oda was in his senior year of high school, he submitted a one-shot for the 44th Tezuka Award -- it would be his public debut as a manga artist. The wild west one-shot was titled "Wanted!" and centered around a good-natured outlaw named Gill Bastar on the run from bounty hunters after his head. This work would later be published in a collection tankebon of Oda's one-shots that was eventually renamed with his debut's original title, "Wanted!"
Oda has also been cited as a fan of Westerns, as one of his favorite movies of all time is Young Guns (1988), a biographical retelling of the life of Billy the Kid, directed by Christopher Cain. Seeing as Oda has a fondness for Westerns, it's fair to say that a future Wild West-themed One Piece arc has legs to stand on.
A European Medieval Fantasy Could Give One Piece More Dragons
One Piece's world is filled with kingdoms, royalty and monarchs, but we've yet to see a more traditional western fantasy kingdom. The Straw Hat Pirates embarking on an island with a kingdom, knights and adventure would certainly not be out of the ordinary given their track record. It would also give Oda an excuse to draw more dragons like the ones in Punk Hazard and the two about to go claw-to-claw in Wano.
The chance to see the Straw Hat Pirates dressed as a D&D adventurer's party would definitely have fans foaming at the mouth. Give Usopp a bow and he could make a great archer. Nami is already a thief, so a hood and cowl is all she needs to be an exceptional rogue, and Brook would make the quintessential undead bard.
The Straw Hats Have Yet to Explore a Subterranean Kingdom
The Straw Hats have traversed the mighty seas, reached the bellowing skies and submerged themselves in the ocean's dark trench, so the last logical landmass for them must be to go underground. And knowing Oda, logically when they eventually do, they will find themselves in a hidden village or city tucked away well below the surface.
Subterranean Cities are a fun setting to explore. It could be a lost advanced civilization like in Disney's Atlantis: The Lost Empire, or an oppressed society controlled by the people above who will rise up against their tyrant similar to Gurren Lagann. The latter would definitely fall in line with Oda's sensibilities, but we could also expect a lot of moles and digging critters given his fondness of animals. If there's any place to drop a Poneglyph for Robin to discover, a hidden society underground would be a perfect location.
A Steampunk Adventure Would Be a Departure for One Piece
A steampunk excursion for our favorite crew would be a radical change of pace for One Piece. In fact, this could be a chance for our more technical members of the crew like Usopp and Franky to get a larger focus. Franky would certainly come up with a new Franky General design, and it's easy to see Usopp tinkering with a plethora of new gadgets. With Brook's gentleman-like demeanor, he could also get some more development with this hypothetical arc.
Neon Skyscraper City Would Lift the Straw Hats to New Heights
The idea of the Straw Hats raising hell in a massive city of neon lights and heavy traffic sounds like chaos waiting to burst off the page. Luffy and the gang could have a blast in a nightclub, have a run-in with a local street gang or crime syndicate, and have a wild car chase to top it off. The only thing setting back this aesthetic is that One Piece isn't set in modern times, but the series is pretty loose when it comes to technological advancements. While aircrafts and non-snail-based communications don't exist in One Piece, we still have cyborgs and advanced bio-genetic engineering. Just give Germa Kingdom and the World Government and few more decades and they'll catch up.