Sasuke could be a time - traveler in a One Piece fanfiction. He gets sent from his own ninja - filled world to the world of One Piece. He has to adapt quickly to the new environment. His goal could be to find a way back home, but in the process, he gets involved with the various power struggles in the One Piece world. He might end up using his Sharingan to help the Straw Hats in battles against the Marines or other pirates.
One plotline could be that Sasuke is from a hidden ninja - like island in the One Piece world. He is sent out on a mission to find a Devil Fruit that has the power to unlock the secrets of his clan's past. Along the way, he crosses paths with the Straw Hat crew and initially clashes with them due to his cold and brooding nature.
Sasuke's character could potentially bring a new level of intensity to the One Piece world. His Sharingan abilities, like seeing through illusions and predicting movements, might make him a formidable opponent. He could be a lone wolf type of character in the Straw Hat crew, initially aloof but gradually warming up to the others, much like Zoro in some ways.
One possible plotline could be Loki trying to redeem himself in the eyes of the Avengers. He has a long history of causing trouble, so it would be a journey of him making amends. Another plotline could be Loki using his knowledge of the Asgardian and other realms to help the Avengers face a threat from those areas. For example, a new powerful enemy from the Nine Realms emerges and Loki's insights are crucial in the fight.
In some fanfictions, Sasuke travels to different hidden villages to learn about their unique cultures and ninja techniques. This allows him to grow as a person and a ninja without any inappropriate 'lemon' elements. Also, there are stories where he mentors the next generation of ninja, passing on his knowledge and experiences in a positive way.
One storyline could be that Sasuke is a fallen Jedi and Sakura is sent to find him. She has to face many challenges in the galaxy to reach him and then convince him to return to the light side. Another could be that they are both on opposite sides of a galactic war. Sasuke is with the evil empire - like the Sith - and Sakura is with the rebels. They have to fight each other but also deal with their feelings.
A great plotline could be Merle Dixon being hired by a wizard to retrieve a powerful magical artifact from a dangerous place. He'd have to team up with some wizards or witches and learn to work with magic along the way. Or perhaps he's a Muggle who has some latent magical ability that gets awakened in the wizarding world, and he has to deal with his new - found powers while also dealing with the prejudices of some pure - blood wizards. He could also get involved in a mystery surrounding an old wizarding family's secrets that has implications for the whole wizarding world.
One possible plotline could be Nico being mistaken for a powerful necromancer from Essos. He gets caught up in the power struggles between different lords and queens who want to use his powers for their own gain. Another could be that he arrives in Westeros just as the threat of the White Walkers is becoming known and he has to decide whether to help the people there or stay out of their affairs.
Well, perhaps the plot could be that Harry discovers a hidden portal that leads to the Naruto universe. There, he meets Sasuke and becomes fascinated with his skills. He decides to learn from Sasuke and in the process, tries to combine magic with ninja techniques.
A plotline could involve an Uchiha character from the Naruto world being sent to the Harry Potter universe. This Uchiha could befriend Harry and teach him some of the Uchiha techniques, like how to enhance his senses or use some basic ninjutsu. Harry, in turn, could introduce the Uchiha to magic. Together, they could face threats in the wizarding world, like a new dark wizard who is immune to normal magic but vulnerable to ninja techniques.
One possible plotline could be Sabo and Koala going on a mission together for the Revolutionary Army. They face various challenges and in the process, their bond deepens. Maybe they have to infiltrate an enemy base and rely on each other's skills to succeed.
A possible romantic plot could be that Sasuke meets a foreign ninja during a diplomatic mission. This ninja is different from anyone he has ever met. She has a unique fighting style and a free - spirited nature. They start off as rivals, constantly trying to outdo each other in training sessions. But as they spend more time together, they begin to understand each other's pasts and motives. Sasuke, who has always been alone, starts to feel a connection. Eventually, they realize their feelings for each other and have to face the challenges of their different homelands and cultures in order to be together.