The tale of One Piece is already a grand enough story just telling the adventures of Monkey D. Luffy. His story alone is one filled with flying ships, singing skeletons, fights with giants, and plenty of memorable characters along the way. However, it is in the latter that the series crosses a huge, missed opportunity.
While the main story itself has plenty of ground to cover, it has brushed over the stories of some of the other characters whose entire histories and internal conflicts could very well build a new series of their own.
10 Donquixote Doflamingo
Donquixote Doflamingo is perhaps the series' most complex villain to date (or, at least, the one that Oda has explored the most). His background is filled with class despair, poverty stricken hardship, and corruption. His childhood alone saw him witness his mother's death, go through horrible torture, and kill his own father.
After that, there's this entire story of him being picked up by Trebol, Pica, and Virgo, starting his own crew as an heir of evil, and conquering an entire kingdom via conspiracy. That doesn't even cover the conflicts with his brother or his grand scheme to get back at the Celestial Dragons.
9 Fisher Tiger
Fisher Tiger is an important figure within the world of One Piece who doesn't nearly get mentioned enough. He was a fisherman who wished to venture to the outside world and learn about the cultures and happenings of man.
However, he not only found a world stricken with corruption, but he himself was captured and sold off as a slave. From there, he would have this entire journey to not only free himself but climb the entire Red Line to save the other slaves of Marie Jois, taking his earned infamy to start his own pirate crew dedicated to freedom.
8 Princess Vivi
Princess Nefertari Vivi's entire childhood is filled with adventures that have way too many gaps to go unnoticed. Her own family is the only one of the original nobles to not move to Marie Jois but instead rule Alabasta personally. Her kingdom is stricken with hardships and poverty yet her father's steadfast confidence helped motivate everyone, until Crocodile came along to seed doubt and descent under his own conspiracy to conquer the country.
After learning of this, Vivi ventured out to try and spy on Crocodile's personal organization from the inside, leading her to start a few friendships with some eccentric bounty hunters. The point between this and her joining the Straw Hats is already filled with so much potential for trials and tribulations that it's aching just to think how she ever survived in the first place.
7 Nico Robin
Nico Robin's own backstory is not only widely accepted as one of the saddest in the series' history but was also a strong instigator for a major pivot in One Piece's sense of storytelling and popularity. Left to a mean, surrogate family while her mother was away fighting as part of the revolution, Nico Robin spent her childhood either doing an entire Cinderella bit or, more interestingly, learning some world secrets and skills from some of the best historians and archaeologists.
After the Buster Call hit Ohara, Robin would begin an entire journey searching for the Poneglyphs while simultaneously joining, infiltrating, and betraying various underworld organizations.
6 Dragon and The Revolutionary Army
There honestly isn't much to say about Dragon and the Revolutionary Army. Other than the fact that they've consistently been referred to as the World Government's #1 threat, not much has actually been revealed about him. Yet, there's still plenty of potential with what has been revealed to let people know that Dragon's journey has been a fun one.
Moving past whatever life he had before and with Luffy, Dragon was responsible for collecting a colorful band of fighters, initially comprised of Kuma the Tyrant and Emporio Ivankov. He'd amass his army and begin an entire rampage across the world, liberating country after country. It would be immensely interesting to see how Dragon came to start the Revolutionaries, found his friends, and conquered the lands that he has. While the story will eventually get to it, nothing may ever really give it the justice that an entire series of its own could.
5 Portgas D. Ace
There's no doubt that Ace stepped unto an entirely different path than Luffy did on his pirate journey. He'd start out his early years as the Captain of the Spade Pirates, gaining enough infamy to be offered a Warlord position. He'd later come across the Whitebeard Pirates and be offered a spot on his crew and, much later, a position as the captain of the 2nd Division.
From there, he'd dedicate his life serving and protecting the man who gave him purpose and, essentially, saved his life. This would involve grand adventures with the Whitebeard pirates, a journey across Wano and a chance meeting with Lil' Oars, Jr., as well as his early hunt for Marshall D. Teach. Not even counting his father issues, Ace's story would really be a refreshing perspective for the world.
4 Gol D. Roger
Considering that his entire legacy is the basis for the current story, Gol D. Roger's story is primed with curiosity and potential. In what could be a very enlightening and nostalgic prequel, Roger's story could look into his formative years, what lead up to him meeting with Rayleigh, the formation of his infamous crew, and what events within his decade-spanning career lead to his incredible infamy.
And, as the series has now revealed, there is this entire second act that lead to Roger finding Laugh Tale as a means of accomplishing his ultimate goal. The material is already there, and a series may very well be coming. All fans have to do is wait for the current one to end to see how it all began.
3 Kozuki Oden
The newest character on this list, Kozuki Oden is absolutely not the least ventured. He was the rebellious heir to the Wano Kingdom whose various escapades within his youth lead to his family disavowing him. He would later undergo a journey through the country's meanest streets and accrue the various misfits that would make up his disciples.
After having easily gotten bored of his newfound power and reclaimed respect from the kingdom, he took the first ship ride out of there and had plenty of adventures with both Whitebeard and Gol D. Roger. All of this passed by in a flash within the manga, making it even more necessary for a dedicated series to flesh all of this out.
2 Trafalgar D. Water Law
One of the biggest newcomers within the series, Trafalgar Law is a pirate who has earned the fan's love, respect, and tears. Everything about him is stained with tragedy. After a disease began to spread among his village and the World Government decided to handle it via genocide, Law would enter the seas of the outside world via carts of dead bodies.
He would later seek out and join Doflamingo's crew, only to be picked up and saved by Doffy's brother, Corazon. After receiving his Devil Fruit, he would embark on an entirely new adventure that would see him hatch a plan to avenge Corazon's death and become takedown Kaido, meet his polar bear and assemble a crew, and invade the ports of a Marine Base with a hundred missing hearts.
1 Roronoa Zoro
Last but not least, this list reaches the Straw Hat member whose own personal story throughout the series has rivaled Luffy's. Zoro's hard working, ambitious drive to become the #1 Swordsmen in the world is an exciting narrative totally independent of Luffy's journey, one filled with an interesting first act, plenty of awesome fights, and an entire training arc with Hawkeye Mihawk.
Seeing his journey or even possibly new adventures after One Piece has ended (possibly being hunted down as the new #1 Swordsmen) would definitely be an interesting avenue that fans would really want to see.