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.
A cool plot for this fanfiction could be that Whitebeard's crew gets combined with another crew from One Piece through some strange magic. They have to learn to work together and face new threats. For example, if they team up with the Straw Hats, it would be interesting to see how their different personalities and abilities interact. Also, what if Whitebeard has to protect a new island that has a power that could change the balance of the One Piece world?
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 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.
Perhaps a villain from DBZ, like Frieza, finds a way to travel to the One Piece universe. He starts enslaving the pirates and taking over islands. Luffy and his crew would be the first to resist, but they quickly realize they need help. Goku and Vegeta then arrive and have to teach the pirates about ki and how to fight against Frieza's powerful minions. It would be a great opportunity to see the different fighting styles blend together.
One idea could be Mihawk getting transported to a different part of the One Piece world where he has to team up with the Straw Hats to face a new and powerful enemy. Maybe a group of ancient sea gods that are awakened and threatening to destroy all pirates. The Straw Hats could initially be wary of Mihawk but soon realize his skills are crucial for their survival.
One idea could be Ace crossing over into a modern - day world where he has to adapt to new technologies while still having his devil fruit powers. He might get involved in some sort of street racing using his fire powers to boost his vehicle.
One idea could be that the characters from Khr end up in the One Piece world and have to navigate the Grand Line. They might join a pirate crew and use their unique powers from Khr in battles against other pirates or the Marines. Another plot could involve a Devil Fruit user from One Piece ending up in the Khr world and causing chaos with their powers there.
A cool plot could involve the Pokemon world's Team Rocket equivalents trying to steal the One Piece. They might think it's some sort of ultimate Pokemon - related power source. Meanwhile, the Pokemon trainers in the One Piece world could form alliances with the pirates. For example, a water - type Pokemon trainer could team up with Sanji on a ship, using their Pokemon's water abilities to help with cooking and other ship - related tasks.
One idea could be that the Straw Hat crew somehow ends up in the world of RWBY. Luffy might be intrigued by the different fighting styles and semblances. For example, he could try to learn how to use aura. Another plot could involve Ruby Rose meeting Nami and being fascinated by her Clima - Tact skills. They could team up to fight a common enemy that threatens both their worlds.
A possible plot is that there's a powerful treasure in the Naruto world that the Straw Hat Pirates hear about. So they sail there. Once they arrive, they meet Naruto and his team. Together, they have to face a common enemy who wants to use the power of both the One Piece and the ninja world for evil. Naruto can teach Luffy and the others some ninja skills like stealth, while Luffy can show Naruto the value of true friendship among his crew. And maybe Zoro could have a sword - fighting duel with a skilled ninja swordsman.
A cool plot could be a powerful gamer who has the ability to control digital elements in the real world. They enter the One Piece universe and team up with the Straw Hats. They create digital fortresses and weapons for the crew during battles. Or perhaps a devil fruit in the One Piece world gives someone the power to enter and manipulate game worlds, and the story follows their adventures between the two realms.