A more complex plot could be Ace crossing into a world full of magic users. Here, his devil fruit power is seen as a strange and unique form of magic. He has to learn about the rules of this new magic system while also dealing with those who want to study or control his power. He might form alliances with some magic users who are outcasts, just like he was in his own world, and together they fight against the establishment that wants to suppress their powers.
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.
A great plot idea could be that the OC is a healer who saves Ace's life after a particularly brutal fight. Ace is initially distrustful but as he recovers, he sees the kindness in the OC. They then go on a journey to find a rare herb that can cure a disease that is spreading among the pirates, and along the way, their relationship blossoms.
One idea could be Ace traveling back to the time when Gol D. Roger was still alive. He could witness the events that led to Roger's surrender and execution, and perhaps even interact with the Pirate King. This would give him new insights into the world of piracy and his own place in it.
One idea could be that Female Luffy and Ace team up for a special treasure hunt. They might discover a long - lost island that holds a powerful Devil Fruit that has unique abilities. Another plot could involve them facing a new and powerful enemy together, and during the battle, they learn more about each other's dreams and pasts. For example, they could find out about a connection between their families that they were previously unaware of.
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.
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 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.
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.
Well, Akainu could cross over into a fan - made version of the One Piece world where the Yonko are all united against the marines. He finds himself in the middle of this conflict and has to decide whether to side with the marines who he used to be a part of or join the Yonko for some self - interested reasons. He could also have a change of heart during this crossover and start to see the pirates' side of things, which would be a huge character development for him considering his strong stance as a marine in the original story.
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.