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.
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.
In a fanfiction scenario where Luffy meets Whitebeard, it could be a really epic encounter. Luffy, being the carefree and determined pirate he is, might immediately show his respect for Whitebeard as a great pirate. Whitebeard, on the other hand, would probably be intrigued by Luffy's unwavering spirit. They could end up sharing stories about their adventures at sea, and perhaps Whitebeard would offer some words of wisdom to Luffy. Maybe they would even team up briefly to take on a common enemy, like some other powerful pirate crew that threatened the peace of the Grand Line.
One great 'One Piece Whitebeard OC fanfiction' is 'The Whitebeard's New Heir'. It has an interesting OC who is a young pirate with a special ability and joins Whitebeard's crew. The story is full of action and adventure as they face off against other pirates.
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.