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.
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.
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.
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 common plot twist could be Ace surviving the battle at Marineford in a way that was unexpected. For example, instead of being killed by Akainu, he could be suddenly transported away by some mysterious power. Another plot twist might be that Ace discovers a new ability or power that he didn't have in the original story, which changes the course of his actions and relationships. Maybe he has a connection to an ancient power in the One Piece world that no one knew about before.
In some fanfictions, a character from Naruto might end up with a Devil Fruit power from One Piece, which is a huge plot twist. For example, Sasuke getting the Logia type Akuma no Mi. It completely changes his fighting style and how he interacts with the world.
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.
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 interesting plot twist could be a character who was thought to be dead returning as a powerful force. For example, a lord who was presumed killed in battle comes back with a new army he raised in secret. Another could be an unexpected alliance between two of the kings, say a northern king and a southern one, due to a common threat that was previously overlooked. And a third might be a discovery of a long - lost heir to one of the kingdoms that throws the whole power struggle into chaos.
One interesting plot twist could be that Peter Parker reveals his identity not because of the Sokovia Accords but to save a loved one from a villain who has specifically targeted him. For example, the villain might have kidnapped Aunt May and demands that Peter show his face to the world in exchange for her safety.
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.