I have read one piece manga but not to the end . But i too consider one piece world very good example of world building . So now there is lot potential stories to be told in one piece world . But no, every fanfiction i have ever read, goes to intersect with story of luffy to somehow and change good story to show their added side character . Why not tell the story of any other person who is not going to participate in original story or living at any other time ? Original story is good enough so why change it ? This also applies to other popular stories . I bet every one of you can count dozens of them .