One Piece's greatest charm lies in its expansive and detailed worldbuilding. The manga series penned by the legendary Eiichiro Oda has been serialized for more than two decades, and both the anime and manga have long surpassed the 1000th release milestone. Despite its long tenure, the series is still to shed light on numerous mysteries, and fans can't get enough of the franchise.
It's no exaggeration to say that the world of One Piece has thousands of named characters and millions of unnamed individuals. Even though the Straw Hat Pirates' journey has already been long and eventful, there are a few characters who have interesting enough narrative to warrant their own series.
Monkey D. Garp's Marine Story
Luffy takes a lot after his grandfather. Both men are reckless muscleheads who follow whatever their conscience dictates. Garp has always wished for Luffy to be a notable Marine officer, but everybody knows what path Luffy has taken. Even though fans will never see Luffy joining the Marines in a thousand years, Garp has proven himself to be a righteous officer time and again through his meritorious deeds. Most of his adventures happened years before Luffy was even born, earning him the title "Hero of the Marines."
Garp's multiple exploits and battles provide more than enough content for Oda to create a series telling his adventures. Should Garp receive his own series, he already has two supporting characters established in former fleet admiral Sengoku and current vice-admiral Tsuru, as well as a friendly rivalry with Gol D. Roger himself. The infamous Rocks Pirates can be the series big bad or at least one of the main antagonists, and the series can also tackle the Marines' hypocrisy by sheltering the vile World Nobles.
Gol D. Roger's Legendary Journey
Gol D. Roger's adventures are literally the stuff of legends. He started the Great Age of Pirates by not only confirming the existence but also revealing the location of the titular legendary treasure. Even though One Piece has already revealed bits and pieces of Roger's journey through Oden's backstory, most of his adventures and discoveries remain a mystery. Plus, Roger had a crew of colorful characters, including the young Shanks and Buggy, who fans of One Piece love today.
Similar to Garp's possible series, Roger has a slew of notable connections, including a rivalry with the Marine hero. Moreover, Roger has a friendly relationship with Whitebeard, as shown when he asked the former Emperor to let Oden join him in his journey. The problem with a possible Gol D. Roger anime or manga series is having great similarities with the current One Piece.
How Joy Boy Started It All
Joy Boy is without a doubt the most mysterious character in the world of One Piece. However, he is also easily the most influential pirate. Joy Boy's existence is first revealed when Nico Robin read the poneglyph in the Sea Forest. It's revealed that Joy Boy is the original owner of One Piece, as well as the person who left everything the world has to offer on the final island of the Grand Line -- Laugh Tale. Everything fans know about Joy Boy is based on the poneglyph's content, which strongly ties him to the events of the Void Century.
A series centered around Joy Boy would not only shed light on the mystery shrouding him but also further expound on One Piece's already vast world. It could serve as a prequel to One Piece as well as feature the ancient kingdoms and what happened to them. Of course, that includes the existence and use of the ancient weapons.
Anime fans laud One Piece for its masterful worldbuilding, especially since the series is yet to answer all the burning questions that fans have about it. Even though another series after One Piece finally ends may be unlikely, these three characters have more possible content to offer.