One idea could be that the male OC is a former slave who was freed by Whitebeard. He has a deep sense of loyalty towards Whitebeard and is always ready to fight for his newfound family. His unique ability could be related to some form of water - based combat, which is rare in the One Piece world.
First, think about his backstory. For example, he could be an orphan who was taken in by the Whitebeard Pirates. This would give him a strong motivation to protect his new family. Then, consider his skills. Maybe he has a superhuman strength that he uses in battles. Also, his personality is important. He could be a joker in the crew, always making everyone laugh during tough times.
One idea could be an OC who has a unique Devil Fruit ability that helps the Whitebeard Pirates in a crucial battle. For example, a fruit that can manipulate the weather in a small area to their advantage.
First, consider the world of One Piece. The OC should fit within the existing power system. So, if your OC has a special ability, it should be on par with what we've seen in the series. For example, not making them overly powerful without any drawbacks.
Well, there could be fanfictions that focus on their epic battles. Maybe ones that explore the backstories of individual crew members in more depth than the original manga or anime. For example, a fanfiction that delves into Thatch's life before joining the crew.
A cool plot would be when the Whitebeard Pirates meet Luffy while he's on a quest to find a rare Devil Fruit. They too are interested in that Devil Fruit for some reason. Instead of fighting over it, they decide to team up. Luffy's unique perspective and the Whitebeard Pirates' experience combine to create an epic journey filled with challenges and new discoveries.
In fanfiction, when the Whitebeard Pirates meet Luffy, they might initially be cautious. Luffy's reputation as a rising star in the pirate world could make them curious. Maybe they would test his strength. For example, some might engage in a friendly sparring match with him to see what he's made of.
In the fanfiction, when the Whitebeard Pirates meet Sabo, there could be a show of strength on both sides at first. However, Sabo's connection to Luffy might make the Whitebeard Pirates more friendly towards him. Ace was part of the Whitebeard Pirates, and since Sabo was like a brother to Ace, they could end up having a big celebration in honor of Ace's memory, with Sabo learning more about his time with the pirates.