One Piece Odyssey is indeed an original story. It builds on the familiar setting and characters but offers a distinct plot that sets it apart from the main manga or anime arcs.
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.
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.
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 some fanfictions, there might be a plot twist regarding the hidden motives of the World Government. They could be revealed to have a much more sinister plan during the Whitebeard War than just taking down pirates. Maybe they are using the war as a cover to test a new and extremely dangerous weapon or to make a power play against other powerful entities in the One Piece universe. This would add a whole new layer of complexity to the story and the relationships between the different factions involved.
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.
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.
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.