One Piece has tons of characters, who are unique in their own ways. Due to the long story, all the characters get an opportunity to show off their power and impress the fans. Like every story, One Piece also has many villainous characters, who have brought harm to others.
While it is true that these characters have a history of being wicked and malicious, there have been instances where they have proved with their actions that they are capable of doing the right thing. So, if given a second chance, these One Piece characters can add another dimension to the story to make things more exciting.
7 Enel
Enel was the main antagonist of the Skypiea arc. After destroying his country, Enel moved to Skypiea with the rest of his officers, and it didn't take long for them to overthrow Gan Fall. Enel was way more powerful than any villain that the Straw Hats had faced.
He had the power of the Goro Goro no Mi, and he combined it with his incredibly powerful Observation Haki, which the Skypieans called "Mantra." Unfortunately for Enel, Luffy's devil fruit was naturally resistant to his powers. This allowed Luffy to defeat the ruler of Skypiea, which resulted in Enel moving to the Moon, where he found many followers. With his newfound followers, it seems that Enel won't be making a return anytime soon, but as there are several mysteries about the winged races that need to be explored, he might visit Earth once again.
6 Bartholomew Kuma
Bartholomew Kuma's journey has been full of ups and downs. Once a revered member of the Shichibukai, Kuma's life started to go downhill after the Summit War. He ended up becoming a mere test subject for Vegapunk's experiments.
Kuma's fall from grace was sad to see; at one point he had single-handedly humiliated the entire Straw Hat crew. Kuma's demise continued, and it reached an all-time low when he was turned into a personal slave by one of the Celestial Dragons. Luckily, he was saved from his plight by Sabo, and he's currently with the Revolutionary Army in the Kamabakka Kingdom. If Kuma has any consciousness remaining, then this could be the second chance that he deserves.
5 Aokiji
Aokiji is a former Navy Admiral, and presently, he is a member of the Blackbeard Pirates. After suffering a defeat at the hands of Akainu, he left the Marines, and he didn't make an appearance until the Punk Hazard arc.
Aokiji has shown time and time again that he is different from the other Marines. He is kind and good-natured, which is a key reason why he helped Nico Robin escape from Ohara. Aokiji has his flaws, but if he were to get another opportunity to join the main cast, it would definitely improve the story.
4 Gecko Moria
Gecko Moria was the main antagonist of the Thriller Bark arc. In the past, Moria used to be rivals with Kaido, but the latter absolutely crushed Moria's crew, which caused him to retreat. Saddened by the loss of his crew, Moria decided to use zombies as crewmates as they would never die.
​When Moria found out about Absalom's possible whereabouts, the former Shichibukai wasted no time in getting there. Unfortunately, Moria was too late to arrive as the Blackbeard Pirates had already killed Absalom, and it made Moria furious. This incident, coupled with the story of his past, makes Moria much more likable. If he does join the Blackbeard Pirates, it would be his second and probably the only chance to impress the fans.
3 Donquixote Doflamingo
Considered to be a malevolent individual, who had absolutely no regard for the lives of any person save for himself, Doflamingo is unquestionably one of the best villains in the series. After living in Mary Geoise for a good amount of time, he was accustomed to certain things, and when his father decided to relocate to another place, Doflamingo was not very pleased when he found out.
Despite his malicious ways, Doflamingo brings a certain charisma that cannot be found in other villains. Ever since he has been locked in Impel Down, there have been multiple attempts on his life, but Magellan has kept him safe. It is possible that Oda might be saving him for a role in the future arcs.
2 Charlotte Katakuri
Charlotte Katakuri is the strongest Sweet Commander of the Big Mom Pirates. He fought against Luffy during the Whole Cake Island arc. The battle between the two characters was incredible as it was not just about winning the fight, there were other things at stake.
The fate of the Straw Hats, Katakuri's responsibility to his siblings, his immaculate record, and overcoming his "ugliness," all these things made it a fight for the ages. Since Big Mom was defeated in Wano, it is highly likely that Katakuri might end up becoming the new captain of the Big Mom Pirates. There is also a possibility that he might join the Straw Hat Grand Fleet as he respects Luffy a lot.
1 The Vinsmoke Family
The Vinsmoke Family made their debut in the Whole Cake Island arc. They were responsible for causing Sanji immense pain when he was a kid. Judge and his three sons absolutely despised Sanji, and they considered him a failure.
Luckily, Reiju helped Sanji to flee, and he started a new life with Zeff. Sanji's past haunted him again in the Whole Cake Island arc, but this time he confronted them without any fear. After Sanji saved them, Judge realized his mistake and helped the Straw Hats to escape from Big Mom's territory. With the family changing so much, it would be interesting to see them get involved in the future again, and see if they can right their wrongs.