One possible plotline could be that through some sort of magical or scientific accident, Naruto ends up in the One Piece universe and Zoro takes him under his wing, and over time Naruto is seen as Zoro's son - like figure. They could go on adventures together and Naruto would learn the ways of the pirates.
One possible plotline could be Naruto discovering his true parentage as the son of Demeter. This could lead to a journey of self - discovery as he tries to understand his new powers related to nature. Maybe he has to hide this from his friends at first because he's not sure how they'll react.
A plotline could involve Naruto's identity being gradually revealed. There could be a lot of shock and disbelief among the villagers. Some might fear his power, thinking he has some hidden agenda as Mito's son. Meanwhile, Naruto would have to deal with his new - found identity and the expectations that come with it. He might also start to explore his connection to the Uzumaki clan more deeply and try to uncover secrets about his mother that were previously hidden. This could lead to all sorts of adventures and conflicts within the Naruto universe.
Maybe the story could start with Nami realizing she's pregnant. This could lead to a lot of mixed reactions from the crew. Zoro might initially be shocked but then become determined to support Nami. There could be a sub - plot where they have to find a safe place for Nami to give birth, away from their enemies. And the crew might have to deal with the idea of a new addition to their family on the ship.
One possible plotline could be that Zoro is kidnapped by an enemy from his past who wants revenge. Maybe a pirate he defeated before has come back with a stronger crew and takes Zoro to lure the rest of the Straw Hats. Another could be that he's kidnapped by a secret organization that wants to use his strength for their own purposes.
One possible plotline could be Naruto discovering his true heritage as Revan's son and having to deal with the new powers that come with it. Maybe he has some latent Force abilities in addition to his chakra.
One possible plotline could be Naruto struggling to come to terms with his new identity as the son of Hachishaku. He might face rejection from some in the village who are suspicious of his new heritage.
A plotline could involve Islamic mythology or figures somehow making their way into the Naruto universe. Maybe an Islamic - inspired spirit or entity appears in the Hidden Leaf Village and has to be dealt with, and the characters learn about Islamic beliefs in the process.
One possible plotline could be Naruto ending up in Alois' world and having to adapt to its different rules and power dynamics. He might use his ninja skills in unexpected ways to survive and make a name for himself.
A possible plotline could be Naruto and his friends being transported to the WW2 era. They then use their ninja skills to try and stop the war. Or perhaps they side with one of the Allied forces and help them in their battles. Another plotline could be that a powerful Naruto villain teams up with a WW2 dictator and they create chaos together. The heroes have to then find a way to defeat this unholy alliance.
A plotline for a Naruto Keyblade fanfic could start with Naruto finding a strange Keyblade - like object during one of his missions. As he tries to figure out what it is, he accidentally activates it and is sent to a different realm. In this realm, he meets other Keyblade wielders who help him understand the power of the Keyblade. Meanwhile, back in the Naruto world, his friends are worried about his disappearance and start searching for him. Naruto then has to master the Keyblade and find a way back home while also dealing with any threats in the new world.