One idea could be a crossover where Naruto's ninja skills are combined with the Kuja Pirates' unique abilities. For example, Naruto could teach some of the pirates his shadow clone jutsu, and in return, the pirates could show him how to use their weaponry more effectively.
Maybe a plot where Naruto is shipwrecked near the Kuja Pirates' territory. He has to learn their ways and help them fight off an enemy that threatens both his world and theirs. This would involve a lot of character development as Naruto adapts to a new culture and forms new alliances.
Another interesting plot could be that the Kuja Pirates accidentally end up in the Naruto world. They get involved in the ninja wars, and Naruto has to figure out whether they are friends or foes. Along the way, there could be cultural clashes, battles, and new friendships formed as the two groups interact and try to understand each other.
You can try some popular fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Naruto Kuja Pirates' on their search bars, and you should be able to find relevant stories.
Maybe the kidnapped person is actually a long - lost heir to a powerful kingdom, and the pirates plan to ransom them back to that kingdom. As the story progresses, the kidnapped character forms an unexpected bond with one of the pirates and they start to plot their escape together while also dealing with the politics of the ransom situation.
A great plot for Beast Pirates OC fanfiction could be an OC who is a doctor with a mysterious past. The OC joins the Beast Pirates under false pretenses, claiming to be able to create a super - soldier serum using Devil Fruit powers. As they work within the group, they secretly try to find a way to cure the madness that seems to be a part of the Beast Pirates' nature. Along the way, the OC has to deal with the various power - hungry and unpredictable members of the pirates, like Jack and Queen, while also hiding their true intentions.
One idea could be that Jake and his crew discover a new part of Neverland filled with magical creatures that can talk and have their own unique abilities. Another could be a time - traveling adventure where they go back to the days when Captain Hook was a young pirate just starting out. Or perhaps they find a mysterious treasure map that leads them to a place where the rules of Neverland don't apply.
One idea could be that Nathan, the pirate, falls in love with a mermaid. He has to overcome the differences between their worlds, like his fear of the deep sea and her distrust of humans. Another could be Nathan falling for a princess on an island he raids, and he then has to choose between his pirate life and love.
One idea could be an OC who has a unique Devil Fruit ability that helps the Whitebeard Pirates in a crucial battle. For example, a fruit that can manipulate the weather in a small area to their advantage.
How about a story where a pony princess has a vision of the pirate world in trouble. She sends a group of ponies as ambassadors. These ponies meet the pirates and they soon find out that a powerful sea monster is terrorizing the pirates. The ponies use their magic to try and defeat the monster. In the process, they form unexpected friendships with the pirates. There could be some really fun scenes of ponies trying to navigate a pirate ship and the pirates being in awe of the ponies' magic.
One idea could be that the RWBY characters find themselves on a cursed ship similar to the ones in Pirates of the Caribbean. Ruby could be the fearless leader, using her scythe in sword - fights on the ship's deck. Weiss might be the one trying to figure out the magic or curses involved, with her knowledge of Glyphs. Blake could use her stealth skills to explore the ship and discover hidden secrets, and Yang would be the muscle, ready to take on any enemy pirates.
One idea could be a crossover with 'Peter Pan'. The pirates could sail to Neverland and have adventures with Peter, Tinker Bell and the Lost Boys. Maybe Captain Jack Sparrow would try to steal the fairy dust from Tinker Bell.
One interesting plot could be that Naruto, with his ability to use chakra and perform ninjutsu, somehow ends up in the world of Pirates of the Caribbean. He might team up with Jack Sparrow. Naruto's speed and strength could help them in their sea adventures and battles against the cursed pirates or sea monsters. For example, he could use his Rasengan to break through the defenses of some powerful pirate ships or fight off the kraken-like creatures with his shadow clone jutsu.