Oda uses different themes for every single island / arc. We had:
- Islands: Sky islands, deep sea islands and prisons, djungle islands, islands on the back of animals, and more.
- Themes: Venice (Water 7), ghosts & zombies (Thriller Bark), Spain's Romantic era (Dressrosa), currently fairy tales & food (Whole Cake Island), and soon samurai & ninjas (Wano).
- Cities: The City of Gold (Skypiea), hidden cities (Zou), sacred cities (Amazon Lily), ...
But there are still settings / themes Oda hasn't used yet. If you think of the settings / themes you know e.g. from other Anime/Manga, movies, fables or video games, which settings / themes / islands / cities would you like to see in One Piece?