One piece has kept the same basic quest throughout the entire series. They just get sidetracked with side quests... Kind of like majoras mask.

And even during the ‘side quests’ they usually manage to tie it together with something related to the main storyline.

Usually some kind of character development, power growth, new crew member, or something that seemed insignificant becoming a huge deal later.

Even One Piece’s side-stories are usually important and shouldn’t be thought of as NOT part of the story. Because One Piece really is about the over-arcing journey to GET to the One Piece. Sure they have a goal, but the fun is in the journey, and the story and characters know this all too well.