Maybe Naruto discovers Cortana's existence while on a mission in a strange dimension. He is intrigued by her digital form and her ability to store vast amounts of information. Cortana, in turn, is fascinated by Naruto's determination and the concept of chakra. They form an unlikely alliance to face new threats that are emerging in both their worlds.
In a more complex scenario, Cortana's code could merge with a chakra - based entity in the Naruto world. This new being would have a unique combination of Cortana's intelligence and the power of chakra. Naruto and his team would have to figure out how to work with this new entity to save their world from a powerful enemy that is using both technology and dark chakra - based magic.
Another concept is that Naruto is sent to Middle - earth. There, he could use his ninja skills and chakra powers. He and Gandalf could team up to fight Sauron. Naruto's ability to make friends and inspire others would be really useful in uniting the different races of Middle - earth against the common enemy.
One idea could be a crossover where Naruto characters gain the abilities of Needless characters. For example, Naruto could get the power of Adam Blade's regeneration. It would be interesting to see how he uses it in battle, especially against tough foes like Pain or Madara.
Another fanfiction concept could be that a few ninjas, including Naruto, are given the ability to transform into mini - Transformers. They could use these new forms to gain an edge in battle, combining their ninja skills with the advanced weaponry and abilities of the Transformers. This would create a whole new dynamic in the Naruto battles and introduce some really interesting fight scenes.
Another concept is that Euphemia discovers a strange portal and steps through it, ending up in the Naruto universe. There, she befriends Naruto and his friends. She could use her diplomatic skills to try and bring peace between the warring ninja clans, while also learning some ninja techniques herself.
One idea could be to have Naruto enter the world of Bloodborne. Maybe he is on a mission and accidentally gets transported there. He would have to deal with the dark and dangerous creatures using his ninja skills. His chakra could be an interesting element in this world, perhaps being mistaken for some sort of otherworldly power by the inhabitants of Bloodborne.
Another fanfiction concept could be that Hachishakusama is somehow summoned into the Naruto world during a battle. Naruto and his friends initially think it's an enemy but then realize it's a being from another dimension. They have to figure out a way to send it back, perhaps by using some ancient sealing jutsu that Naruto has to learn specifically for this task.
Another possibility is that Godzilla somehow ends up in the Naruto world. The shinobi might initially see it as a great threat, but Naruto could try to communicate with it using his ability to sense emotions. Maybe Godzilla is there because it's being chased by an even greater enemy from its own world, and Naruto and his friends decide to help it.
Maybe a story where the Akatsuki members from Naruto are involved in the Kakegurui school's gambling underworld. They could be using their unique powers and abilities to gain an edge in the games. Sasuke might use his Sharingan to read his opponents' tells during a card game, adding a new level of intrigue to the Kakegurui concept.
In a Naruto - For Honor fanfiction, the Akatsuki members could be a part of the story. They could form a sort of mercenary group within the For Honor world. Their unique abilities and personalities would make for interesting interactions with the native For Honor characters, like the Samurai and Vikings.
Maybe a story where Revy is sent to the Naruto universe. She could be seen as a strange and powerful outsider. Her combat skills with guns would be a unique addition to the battles in the Naruto world. And perhaps she would form an unlikely alliance with some of the ninja clans.