Well, one unique feature is the blend of different universes. For example, in a Naruto and Harry Potter crossover, Naruto's ninja magic can be compared to the wizardry in Harry Potter. It creates a whole new set of rules and interactions.
Another good one is 'Naruto's Childhood in the One Piece World'. Here, Naruto as a kid finds himself on the Grand Line. He befriends Luffy and the Straw Hat crew. The story is full of humor as Naruto tries to understand the ways of the pirates while also showing off his ninja abilities.
Perhaps the kid is from a foreign land that Naruto visited during one of his missions. The kid could be an outcast in his own place because of some unique trait. Naruto, being the kind - hearted person he is, decides to adopt him. It could be that the kid has a different kind of chakra or a strange mark on his body that makes others afraid of him, but Naruto doesn't care.
Naruto himself could be a great crossover character. Since Kakashi is his mentor in the original series, seeing them interact when Kakashi is a kid could be really interesting. They could have a different kind of dynamic, with Naruto perhaps being the more experienced one in some aspects. Another potential character is Goku from Dragon Ball. The contrast between Kakashi's ninja skills and Goku's martial arts and superpowers would create a really exciting crossover.
Character growth is also a big theme. In crossovers, Kid often has to learn new ways of thinking and fighting. For instance, if he's in a world with superpowers like in the My Hero Academia crossover, he has to figure out how to use his skills alongside those new powers. And of course, friendship is a common theme. He forms new bonds with characters from different universes, which helps him develop as a character and also makes for some heartwarming moments in the fanfiction.
One common theme is the clash of cultures. For example, when Sasuke is in a different world like the world of Western superheroes, his ninja upbringing and values contrast with the local ones. This creates interesting conflicts and character development.
If we consider a crossover with the Bleach universe, Ichigo could be paired with Kid. Ichigo is a soul reaper with a strong sense of justice. Kid, on the other hand, has his own pirate code. They could cross paths when some Hollows start attacking Kid's ship. At first, Ichigo might see Kid as a threat, but as they get to know each other, they could work together to defeat the Hollows.
A Naruto and Albedo fanfiction crossover could be really interesting. It could start with Naruto somehow ending up in the world where Albedo exists. Maybe he gets there through a strange jutsu gone wrong. Naruto's positive and determined personality could contrast with Albedo's more complex nature. They could team up to face a common enemy, with Naruto's combat skills and Albedo's knowledge of her world combining to create a unique dynamic.
In a Naruto - Skullgirls fanfiction, Sakura could team up with one of the Skullgirls. Sakura's medical ninjutsu combined with the brute force of a Skullgirl character would make a formidable duo. They could go on adventures together, facing off against common enemies. It would also be fun to see how the two different art styles blend in the fanfiction, like Naruto's more traditional anime look with the unique, somewhat gothic style of Skullgirls.
One possible scenario in a Naruto - Kaneki fanfiction could be that Kaneki somehow ends up in the Naruto world. Maybe through a strange portal or some sort of jutsu gone wrong. He might initially struggle to adapt to the different environment and the presence of chakra. But his own unique kagune abilities could be seen as a strange new form of power in the Naruto universe.