One idea could be a character in the Naruto universe who mysteriously obtains the Geass bloodline. This could lead to a whole new set of powers and conflicts. For example, they might use the Geass power to control powerful ninja beasts or even other ninjas in the heat of battle, creating a moral dilemma as Geass often comes with a price.
A fanfiction could focus on a group of ninja who are descendants of a long - lost clan that had the Geass bloodline. They are shunned by the other ninja clans because of their strange power. They then set out on a journey to prove that they can use their Geass power for good in the Naruto world, which is filled with its own set of power struggles and politics.
One idea could be a crossover where the Sharingan in Naruto has some similarities to the Geass in Code Geass. For example, a character from Naruto could gain a Geass - like power that is related to their existing ninja abilities. Maybe Sasuke could get a Geass that allows him to instantly copy any jutsu he sees, just like his Sharingan but with an added power boost.
One possibility is that the unique powers from Naruto like ninjutsu could be combined with Digimon's digital abilities. For example, a character with a Sharingan bloodline might be able to analyze a Digimon's attacks more effectively and use that knowledge to counter. Another aspect could be the integration of Digimon evolution with the growth and power - up in Naruto. Maybe a character with a special bloodline can form a stronger bond with a Digimon, leading to more powerful evolutions. Also, the different bloodlines in Naruto could give certain characters an edge in the Digimon world, like the Uzumaki's strong life force helping in battles against powerful Digimon.
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.
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.
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.