One idea could be a crossover where the characters from Monster High end up in the Naruto universe. For example, Draculaura could use her vampire powers in a unique way during a ninja battle. Maybe she can turn into a bat to avoid attacks or use her charm ability on some of the ninja opponents.
Another concept could be that some of the Naruto characters are sent to Monster High. Naruto himself might be really confused by the whole monster culture at first. He could try to use his ninja skills to fit in, like using his shadow clone jutsu to attend multiple classes at once. And his positive attitude could start to change the way some of the Monster High students view outsiders.
One idea could be that Naruto is somehow transported to the world of High School DxD and meets Moka. He could use his ninja skills in the battles that take place in the DxD world. Maybe he teams up with Moka to fight off common enemies.
One idea could be a crossover where Naruto is somehow transported to the world of High School DxD. He meets Gabriel, and they team up to face a common threat. Maybe a powerful devil or a new, unknown enemy. Naruto's ninja skills could be a great asset in the battles of the DxD world.
Another idea is that Naruto is reborn in the High School DxD world as a half - dragon due to the influence of the Dragon God. He then has to navigate the complex social and power structures of the DxD world while also hiding his true identity at first. As he discovers more about his new powers and the world around him, he gradually becomes a key figure in the battles and politics of the DxD world.
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.