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.
In fanfiction, the death of Ace and Whitebeard could lead to a new era of piracy. The crew members left behind might form new alliances or go their separate ways. Some might become more ruthless in their pursuit of power, while others might try to uphold the values that Whitebeard taught. The Marines might also take advantage of the situation, tightening their grip on the seas and trying to root out any remaining pirates associated with Whitebeard.
Another theme is the search for family. Since Whitebeard's crew is like a big family, the OCs often find a sense of belonging within the crew. They might have had a rough past without a proper family, but with Whitebeard's crew, they finally have a place to call home. This is explored in a lot of fanfictions through the OC's journey of self - discovery and acceptance within the crew.
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.
It could create a huge rift among the pirate crews. Luffy's friends and crewmates would surely go after him, and this might lead to battles between different pirate groups. For example, the Straw Hats would not sit idly by when their captain is kidnapped, and they might clash with Whitebeard's crew, changing the power dynamics in the One Piece universe.
The meeting between Luffy and Whitebeard in a fanfiction can have a huge impact on the story. It could lead to new alliances. For example, if Luffy and Whitebeard form a bond, their crews might work together in future battles. This would change the power dynamics in the One Piece world. Also, Luffy could gain new skills or knowledge from Whitebeard. Maybe Whitebeard teaches him a new way to use his Haki, which would make Luffy even stronger. It could also inspire Luffy to think about his crew in a different way, like how Whitebeard values and protects his own crew.
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.
Another option for the main antagonist could be a corrupted version of one of Whitebeard's own crew members. This character could have been influenced by some dark power and now wants to take over Whitebeard's position. They would know all of Whitebeard's weaknesses and strategies, making them a very formidable foe for the crew.
In many Whitebeard War fanfictions, Marco the Phoenix stands out. His ability to heal and his loyalty to Whitebeard are remarkable. Also, Squard is often a memorable character. His betrayal and then possible redemption can be a very interesting arc within the fanfiction. He adds an element of internal conflict within the Whitebeard Pirates which makes the story more complex.
One idea could be that Whitebeard is transported to a different time in the One Piece world. Maybe he ends up in the future and has to deal with new pirates and powers that he's never seen before. Another could be a crossover with another anime world, like he somehow gets stuck in the Naruto world and has to find his way back while dealing with ninjas.