One potential crossover character could be Harry Potter. Imagine Naruto, with his immortality, meeting Harry in the wizarding world. Naruto could teach Harry some ninja skills, and they could fight together against Voldemort. Another could be Superman. The contrast between Superman's superpowers and Naruto's ninja skills, along with Naruto's immortality, would make for an interesting story.
Obviously Naruto himself is a central character. His popularity from the original series brings a lot of attention. Then there might be characters from the crossover world that gain popularity. For example, if it's a crossover with a Final Fantasy world, characters like Cloud or Squall might be popular in the fanfiction.
There could be a really interesting fanfiction where Naruto characters possess immortal souls like in Soul Eater. For example, Naruto's strong will could be translated into an extremely powerful and unbreakable immortal soul. The ninja world's chakra system might interact with the concept of souls from Soul Eater in unique ways, like using chakra to enhance or protect the soul. Maybe Sasuke's Sharingan could have a new ability related to seeing and manipulating the immortal souls.
Goku from Dragon Ball could be a great crossover character. His Saiyan powers and love for combat could contrast and interact in an interesting way with Naruto's ninja skills and immortal sage abilities. They could have epic battles or team up against a common enemy.
One interesting concept could be how Naruto's ninja skills interact with the new elements introduced in the crossover. For example, if it's a crossover with a magic - based world, how he adapts his chakra control to magic spells.
Well, in this fanfiction, Naruto's immortality could lead to him being a sort of legend in the Pokemon world. He might start a new ninja - Pokemon fighting style. He'd use his shadow clones to train multiple Pokemon at once. And his immortality would mean he could face any danger in the Pokemon world without fear of death. This could attract the attention of the Pokemon League, who would be both intrigued and a bit worried about this new and powerful trainer.
In a crossover with Attack on Titan, Eren could be paired with Naruto. Naruto's immortality would give him an edge in the fight against the Titans. He could teach Eren some of his ninja techniques, like how to move stealthily. And Eren's determination to protect his people could be something Naruto relates to. They could work together to find a more permanent solution to the Titan problem, rather than just fighting them constantly.
The interactions would be really interesting. Naruto's immortality gives him a different perspective. He might not take losses in Pokemon battles as seriously as others. He could cheer up a sad Pokemon with his positive attitude. When it comes to trainers, he'd be a bit of an enigma. Some might be intimidated by his immortality and ninja powers, while others would be eager to be his friend. He could also introduce some Naruto - style training methods to the Pokemon world, like making Pokemon do tree - climbing exercises similar to what he did in his ninja training.
One idea could be Naruto being immortal and crossing over into the world of Harry Potter. He could use his ninja skills to navigate the wizarding world, like using his shadow clone jutsu to attend multiple classes at Hogwarts. Another plot could be in a Final Fantasy crossover, where Naruto's immortality makes him a key figure in a battle against a powerful, world - destroying entity. His never - ending life gives him the time to master different magics and skills from the FF universe.