A major theme is the combination of magic and ninjutsu. Since both Naruto and Harry Potter have their own forms of magic, in yaoi fanfiction, seeing how these magical elements blend in the context of relationships is common. There's also the theme of acceptance. The characters may have to deal with the fact that their yaoi relationships are different from what is considered normal in their respective worlds. And the theme of adventure often comes in, as they go through various challenges together in a relationship that spans two very different fictional settings.
Websites like Archive of Our Own (AO3) are great places to start. It has a large collection of fanfiction, including Naruto and Harry Potter crossover yaoi stories. You can use the search function to narrow down your results.
There's 'NaruHarry: Cross - World Connection'. This fanfiction explores how Naruto and Harry meet through some sort of magical or interdimensional event. It shows their growth together as they face challenges from both the wizarding world and the ninja world, with their relationship at the center of the story.
One common theme is the clash and combination of different powers. Harry has magic from the wizarding world and Naruto has chakra - based ninja abilities. Another theme is the exploration of different cultures. The wizarding culture of Harry Potter and the ninja culture of Naruto are quite distinct. And of course, the yaoi theme focuses on the romantic relationship between male characters from the two series.
In 'Harry Potter Incubus Yaoi' fanfiction, it often blends by inserting the concept of incubus (a type of demon) into the Harry Potter universe. The yaoi aspect might come in through male - male relationships. For example, an incubus character could be introduced into Hogwarts and form a relationship with a male character from the original series, thus combining the magical world of Harry Potter with the otherworldly and often sexualized nature of incubus and the male - male relationship dynamic of yaoi.
One great fem Harry Potter crossover Naruto fanfiction is 'The Potter-Namikaze'. It combines the magic of the Harry Potter world with the ninja skills of Naruto. The story follows a female Harry Potter who somehow ends up in the Naruto universe and has to adapt. It's full of interesting character interactions and unique plot twists.
One popular 'Naruto Harry Potter yaoi fanfiction' is 'The Magic of Shinobi Bonds'. It weaves an interesting story where characters from both universes interact in unique ways. Another is 'Cross - Realm Alliances' which has some really creative plotlines involving the two different worlds and their characters in yaoi relationships.
One idea could be that Naruto's immortality is due to some sort of ancient ninja magic that has parallels to the magic in the Harry Potter universe. For example, he could have made a pact with a powerful magical creature in the Naruto world that gives him eternal life, and when he enters the Harry Potter world, he has to figure out how this magic works there. Maybe he tries to find a way to share his immortality with his new friends in Hogwarts, like Harry, Hermione, and Ron. He could also use his ninja skills along with his immortality to protect the wizarding world from new threats that the regular wizards are not equipped to handle.
One common theme is power dynamics. Since Naruto has powerful ninjas and when crossed over with other universes in a yaoi context, the differences in power can create interesting relationship dynamics.
One popular Harry Potter x Naruto yaoi fanfiction is 'Crossing Realms of Magic and Ninjutsu'. It explores an interesting world where the two universes collide and the characters develop unique relationships. However, it's important to note that fanfiction can vary greatly in quality and content.