A major theme is identity. Naruto has to re - evaluate who he is in a new and often very different universe. His immortality also makes him a bit of an anomaly, so themes of acceptance and being different are also common. For instance, in a crossover with a high - tech universe, his more traditional ninja skills and immortal state make him stand out. He has to find his place among characters who might not understand his abilities or his immortality at first.
One great 'immortal naruto crossover fanfic' is 'Naruto in the World of Highlander'. In this fic, Naruto is thrust into the Highlander universe where immortals exist. He has to adapt to their rules and use his ninja skills. Another one could be 'Immortal Naruto and the Dragon Ball Universe Crossover'. Here, Naruto's immortality plays a key role when he interacts with the powerful characters in the Dragon Ball world.
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.
A possible fanfic could involve a fusion of the Force and chakra. Vader discovers the existence of chakra in the Naruto world and tries to combine it with the Force for his own purposes. Naruto and his allies must stop him while also being intrigued by the similarities and differences between the two powers.
A Kim Possible and Naruto fanfic crossover could be really interesting. It could bring together the unique worlds of Kim Possible, with her high - school spy adventures, and Naruto's ninja world. Maybe Kim could use her gadgets in the Naruto universe to help out the ninja, or Naruto could teach Kim some of his ninja skills. There are so many possibilities for a great story here.
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.
Aang from Avatar: The Last Airbender would be an excellent choice. Aang has the ability to bend all four elements, and his connection to the spiritual world could offer Naruto new perspectives on his own chakra. Another good one is Natsu from Fairy Tail. Natsu's dragon - slayer magic and his intense fighting spirit would make for an interesting training partner for Naruto. Additionally, Midoriya from My Hero Academia. Midoriya's analysis of his opponents and his own growth in power could be something Naruto could learn from.
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.
One interesting aspect is seeing how different universes interact. For example, if it's a 'Naruto' and 'Dragon Ball' crossover not focused on Naruto, we can see how the power - scaling of 'Dragon Ball' characters might be affected by the presence of 'Naruto' elements. It's like a new playground for the 'Dragon Ball' characters.