One idea could be a crossover where a character from the Harry Potter world discovers the Kusanagi no Tsurugi. Maybe Harry himself stumbles upon it during a magical journey outside of Hogwarts. The sword's magic could interact with his own magic in unexpected ways.
The idea of 'Harry Potter uses Kusanagi' in fanfiction is likely a product of the writer's imagination to break the boundaries of the original Harry Potter world. Kusanagi has its own set of legends and properties. By having Harry use it, the fanfiction author might be trying to show how Harry can adapt to and utilize something completely foreign. It could lead to all sorts of interesting scenarios, like battles against new enemies or the exploration of uncharted magical territories.
One possible plot could be that Harry Potter, through some magical mishap, ends up in a world where Kusanagi exists. He might use his magic to help Kusanagi in a battle against a common enemy. For example, Harry could use spells like 'Expelliarmus' to disarm Kusanagi's foes.
Harry Potter and Kusanagi are the central characters. Harry, being used to the magical society of Hogwarts, would have to adapt to the world of Kusanagi. And Kusanagi would have to come to terms with the existence of magic in the way Harry uses it. There could also be other supporting characters from both their worlds. Maybe some of Harry's friends from Hogwarts like Hermione could make an appearance and offer her vast knowledge to help in the situation. Or there could be characters from Kusanagi's side who are initially skeptical of Harry but later come to respect him as they see his powers at work.
Begin by imagining how Kusanagi would fit into the 'Harry Potter' world. Consider her appearance, her abilities, and her personality. For example, if she has some sort of sword - like in the name 'Kusanagi' which might be related to a famous sword in some mythologies - how would that interact with the magic in the 'Harry Potter' world? Maybe she can use it to deflect spells. Then, start building a story around her relationship with Harry Potter and the other characters. You can start with a simple idea like Kusanagi being invited to Hogwarts for an exchange program and then develop the story from there.
One interesting 'Harry Potter Kusanagi' fanfiction could be one where Kusanagi, with her unique skills, teams up with Harry Potter to face a new dark wizard threat. Maybe she uses her knowledge of Japanese magic (assuming she has some in the fanfic) to complement Harry's Western magic.
Well, Hermione might study the Kusanagi no Tsurugi. She'd be fascinated by its history and magic. She could try to decipher any inscriptions on it using her vast knowledge of ancient languages. Maybe she'd find a connection between the magic in the sword and some of the lost spells in the wizarding world.
I'm not sure if there are extremely popular ones specifically with that exact concept. Fanfiction is a vast world and while there might be some that incorporate this idea, it may not be mainstream. However, it could be a hidden gem in the fanfiction community that only a few dedicated readers have discovered.
In Naruto fanfiction, the Kusanagi sword often takes on a significant role. It might be a powerful weapon that Naruto discovers and has to master. For example, it could be hidden in an ancient temple and Naruto, through his adventurous spirit, stumbles upon it. Then he has to learn how to use it in combat against new and powerful enemies.
A fanfiction about Naruto getting the Kusanagi could also involve a time - travel aspect. Naruto could be sent back in time and end up in the era when the Kusanagi was first created. There, he meets the original wielder and learns from them. He then brings the knowledge and the sword back to his own time, using it to defeat a great evil that was previously unbeatable.