One idea could be a story where Sanji and Usopp get stranded on a strange island together. Sanji has to rely on Usopp's creativity to find food sources that are not immediately obvious, while Usopp depends on Sanji's combat skills when they encounter dangerous creatures on the island. This would force them to bond and understand each other better.
One idea could be that Sanji and Usopp team up for a cooking competition on the ship. Sanji brings his world - class cooking skills and Usopp uses his creativity to come up with unique presentation ideas. Another could be a situation where they get lost on an island together and have to rely on each other's skills, Sanji for survival cooking and Usopp for building shelters and creating distractions against potential threats.
Well, he could time - travel to a point where the Straw Hats first formed but he's the only one who remembers the future. This would lead to some really funny and interesting situations. He might try to prevent some of the early hardships they faced, like the fight with Buggy. Or perhaps he travels to a time during the Void Century and discovers secrets that could change the course of the current One Piece world. He then has to figure out how to bring this knowledge back to his own time without causing chaos.
Perhaps there's a mission where Sanji and Usopp have to go undercover as a couple in a high - society event. At first, it's just for the mission, but as they play their parts, they start to feel real emotions. They have to navigate through the social intrigue, all while dealing with their own growing feelings for each other. This situation could lead to a lot of cute and romantic moments in the fanfiction.
Sanji's chivalrous nature can be further explored. For example, when he and Usopp are in a dangerous situation, Sanji could put himself in harm's way to protect Usopp, which shows his loyalty and sense of honor. Usopp, on the other hand, can develop his courage. Maybe he stands up to a powerful enemy to protect Sanji, overcoming his usual cowardice and growing as a character.
One common theme is friendship development. Since they have different personalities, seeing how they become closer and understand each other better is often explored. For example, Sanji's chivalry and Usopp's cowardice at times can create interesting dynamics as they learn to accept and support each other.
One idea could be that Naruto and Luffy meet in a strange dimension. Naruto's ninjutsu and Luffy's devil fruit powers could create really cool battle scenes against a common enemy. For example, they might face a powerful being who can control time, and Naruto's shadow clone jutsu combined with Luffy's stretching ability would be used in creative ways to defeat it.
Maybe in the fanfiction, they get stranded on an island. Fem Zoro tries to find a way off while Sanji discovers rare ingredients on the island. As they spend time together, they start to understand each other better. Sanji realizes Fem Zoro's determination is just as strong as a man's, and Fem Zoro sees Sanji's kindness beyond his flirty nature.
One great sanji x usopp fanfiction is 'The Unlikely Bond'. It beautifully explores their relationship in a world where they have to face a new and dangerous enemy together. The story shows how Sanji's cooking skills and Usopp's inventiveness complement each other in unexpected ways.
One interesting plotline could be Zoro getting injured during a battle and Sanji having to take care of him. This would lead to a lot of bickering at first but then slowly developing into deeper feelings as they spend time together. Another could be them getting stranded on an island and having to rely on each other for survival, which forces them to confront their feelings for each other. And a third could be a situation where they are both targeted by a powerful enemy because of their relationship, and they have to fight together to protect their newfound bond.
One idea could be that Luffy and his crew end up in the Naruto world. They meet Naruto and his friends, and together they have to face a new common enemy. Maybe an Akatsuki member teams up with a powerful pirate from One Piece's world to try and take over both realms.