For a more complex crossover, consider Jiraiya in the world of Harry Potter. He could be an exchange wizard from a different magical system. He might introduce some ninja - like magic to Hogwarts, and his toad summons could be a really strange sight in the Forbidden Forest.
One idea could be a crossover with Naruto in a different timeline where Jiraiya survives and teams up with Naruto to face a new, even more powerful enemy from another dimension. They could combine their unique jutsu in unexpected ways.
One plot idea could be Jiraiya training Kushina. Since he was a great ninja and had a lot of knowledge, he could pass on some unique jutsu or survival skills to her. This would not only strengthen Kushina but also create a bond between them based on mentorship.
One idea could be that Jiraiya focuses on Naruto's rasengan training in a more extreme way. He takes Naruto to a secluded place full of natural challenges like strong water currents or rocky terrains. There, Naruto has to constantly adapt and use his rasengan in different situations to overcome obstacles. Another plot could involve Jiraiya teaching Naruto some secret clan jutsus that he discovered during his travels, which could give Naruto an edge in future battles. And also, Jiraiya could train Naruto in stealth and espionage skills, which are essential for a shinobi but Naruto hasn't had much focus on before.
Well, perhaps there could be a plot where they both get trapped in a mysterious dimension. In this dimension, they have to rely on each other's skills. Jiraiya's knowledge of sage mode and Orochimaru's various jutsu could be combined to find a way out. It could also explore their relationship in a more in - depth way as they face different challenges together.
One interesting idea could be a crossover with "Moana". Elsa's ice powers and Moana's connection to the ocean could create a really cool dynamic. Maybe Elsa accidentally freezes part of Moana's ocean and they have to work together to fix it. Another could be with "Tangled". Rapunzel's long hair and her adventurous spirit combined with Elsa and Anna's journey in Frozen 2 would be exciting. Rapunzel could use her hair to help them climb mountains or cross chasms. A third idea is a crossover with "Beauty and the Beast". Belle's love for books and knowledge could help Elsa understand more about her powers and the history of magic in the Frozen world.
One plot idea could be that Hinata discovers a lost scroll of Jiraiya's that contains a unique and powerful jutsu. She then tries to find a way to return it to him, and along the way, they form an unexpected friendship. Jiraiya could see her determination and offer to train her in some aspects of the jutsu.
A plot could involve Tsunade and Jiraiya in their younger days. Maybe they both had a crush on each other but were too shy or focused on their ninja training to admit it. As they grow older and more experienced, their feelings resurface. Tsunade might start to notice how Jiraiya has always been there for her, through her losses and tough times, and this realization blossoms into love.
Another interesting plot would be that Jiraiya is tasked with protecting Naruko from a powerful and shadowy organization. This organization believes that Naruko has some kind of power that could disrupt their plans. So Jiraiya has to use all his skills and knowledge to keep Naruko safe. As they are on the run, Naruko also discovers new aspects of her own strength and the bond between them grows stronger.
A Rambo crossover fanfiction could involve him crossing over into the Star Wars universe. Rambo's guerilla warfare tactics could be interesting in a galaxy far, far away. Maybe he helps the Rebel Alliance in a ground - based battle against the Empire. Or a crossover with the Predator series. Rambo would be a worthy opponent for the Predator, and the fight between them in the jungle could be epic. Also, a Rambo and John Wick crossover would be intense, two highly skilled fighters on the same side or perhaps initially at odds.
One idea could be a crossover with a modern military setting. The Decepticons could be trying to steal advanced military technology. Another is a crossover with a post - apocalyptic world where they are competing with other factions for resources. A third could be a crossover with a magical realm, with the Decepticons trying to harness the magic for their own evil purposes.
One idea could be a crossover with Naruto. Kakuzu could be sent to the Naruto universe and form an unlikely alliance with a shinobi village to fight a common enemy, like the Akatsuki from his own world turned rogue in the new universe. This would create a lot of interesting character dynamics and battles.