Well, they might be really surprised by the unique and wild world of One Piece. The RWBY characters are used to their own world with Grimm and Dust, while One Piece has Devil Fruits and the Grand Line. They could be shocked by the powers of the Straw Hat Pirates like Luffy's stretching ability. Maybe they'd be inspired by the strong bonds between the One Piece characters, which could make them reflect on their own relationships in RWBY.
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 RWBY characters' opinions would vary greatly. Ruby, as the team's eager beaver, would be all in for the One Piece fanfiction. She'd be like a kid in a candy store, loving every new discovery. Weiss, being a bit of a snob sometimes, would have a more measured approach. She might nitpick about things like the logic of Devil Fruits or the pirates' lack of proper decorum. Blake would find kinship in the outcasts and rebels in One Piece, and her views would be colored by that. Yang would be the one shouting at the page during the intense fight scenes, really getting into the action. So, in summary, their reactions are as different as their personalities.
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.
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 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.
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.
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.