One idea could be a crossover with Naruto. Picture a character who follows Jashin in the Naruto universe and how they interact with the existing ninja clans. They might bring a new form of combat or ideology that challenges the shinobi way.
Another possibility is a crossover with a fantasy world like Lord of the Rings. A follower of Jashin could be an outcast in Middle - earth, trying to spread their faith. This would create a lot of conflict with the good - natured races like the elves and hobbits who uphold different values.
One idea could be that your OC is a new recruit to the Jashin cult. They start off being naive about the true nature of Jashin's teachings but gradually discover the dark and extreme aspects as they get more involved. For example, they could be sent on a mission to sacrifice a powerful being to prove their loyalty.
Many Jashin Naruto fanfictions explore the idea of a Jashin - influenced character joining Naruto's side. This creates an interesting dynamic as they have to reconcile their Jashin - related beliefs and practices with Naruto's ideals of friendship and peace. It could lead to internal struggles for the character and also force Naruto and his friends to question their own beliefs about different cultures and religions in the ninja world.
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 interesting 'naruto jashin fanfiction' story could be about a character who is trying to break free from the dark influence of Jashin while in the Naruto world. For example, a ninja who was forced to follow Jashin's ways finds a new path through an unexpected friendship with Naruto.
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.
Another option is a crossover with 'The Little Mermaid'. The characters from the descendants could meet Ariel and her friends underwater. Evie might be interested in the mermaid fashion and could design new outfits inspired by the underwater world.
Another possibility is a crossover with the world of Naruto. The characters from MHA could participate in the Chunin Exams. Bakugo might be really into showing off his skills in this new and different kind of competition.
One idea could be a Venom crossover with Spider - Man in an alternate universe where they have to team up against a common enemy like Carnage. Another could be a Venom and Deadpool crossover, with their crazy personalities clashing and then somehow working together. Maybe a Venom crossover with the X - Men, where Venom's alien nature causes some interesting reactions among the mutants.