Well, perhaps there's a prophecy in both worlds about a group that combines the strength of ninjas and pirates. Naruto's group and Luffy's crew then come together believing they are the ones. They have to work together to unlock the secrets of an ancient power. In the process, they face many challenges. For instance, the Straw Hats have to deal with the complex political situations in the ninja villages, and Naruto's team has to get used to the strange sea creatures in the One Piece world.
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.
One idea could be that Naruto and Luffy's crews end up on a strange island. There, they meet a group of powerful and alluring female characters. The females have unique powers related to both the Naruto and One Piece universes. For example, some can use chakra and also have Devil Fruit abilities. The two crews have to team up with these women to face a common enemy, and during their adventures, romantic relationships start to form in this harem - like situation.
One idea could be that Naruto, with his newfound immortality, somehow gets transported to the One Piece world. He could team up with Luffy and his crew. Maybe Naruto's jutsu would be mistaken for some kind of devil fruit power. For example, his shadow clone jutsu could be seen as a really strange form of duplication devil fruit ability. Another plot could involve Naruto helping the Straw Hats fight against a powerful enemy that has powers similar to those of the Akatsuki in Naruto's world. He could use his immortality to outlast the enemy in a long - drawn - out battle and find a weakness.
One idea could be that the Straw Hats end up in the Naruto world somehow. Maybe Luffy and Naruto meet and instantly become friends because of their similar positive and determined personalities. They could team up to fight a common enemy like Akainu who has somehow also entered the Naruto world and is causing chaos. Another plot could be that the Devil Fruits from One Piece start to appear in the Naruto world, and ninja start to use them in combination with their jutsu, creating new and powerful abilities.
One idea could be that the Straw Hats end up in the Naruto world somehow. Maybe they land on the Hidden Leaf Village's outskirts. Luffy could befriend Naruto immediately because of their similar natures. Naruto would be intrigued by Luffy's Devil Fruit powers and Luffy would be amazed by Naruto's jutsu. Another idea is a battle tournament where characters from both universes participate. Zoro could face off against Sasuke, which would be an epic sword - against - sword and Sharingan - against - three - sword - style fight.
Another concept is that the Devil Fruits in One Piece could be related to the chakra system in Naruto. For example, a character from Naruto accidentally eats a Devil Fruit and has to adapt to this new power. Or perhaps the One Piece itself is a powerful object that can also be used to control chakra in some way. The story could follow the characters as they explore this connection and deal with the chaos it causes in both worlds.
Perhaps in this crossover, the Akatsuki from Naruto are after a Devil Fruit in the One Piece world. Naruto and his friends team up with the Straw Hat Pirates to protect it. Along the way, there would be a lot of cultural clashes and misunderstandings that lead to both comical and heartwarming moments. For example, Sasuke might initially not trust the pirates but then come to respect them as they fight side by side.
One idea could be that Naruto and his friends somehow end up in the One Piece world. Naruto's ninja skills like shadow clones could be really useful in pirate battles. For example, he could use the clones to confuse the enemy pirates while Luffy and his crew attack. Another idea is that the Devil Fruits in One Piece could be incorporated into the Naruto universe. Maybe some ninjas start to eat Devil Fruits and gain new powers, which would create a whole new dynamic in the ninja world.
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.
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.