Another idea is a story where a character from the Chainsaw Man universe ends up in the Naruto world. This could lead to all sorts of chaos as they introduce their unique abilities and concepts. For example, they might try to teach Naruto and his friends about devil contracts in the context of the Naruto world's power system.
Perhaps in the fanfiction, Naruto discovers an ancient weapon that is a chainsaw-like device with chakra - based powers. He has to learn how to master this new weapon while also dealing with the political implications of such a powerful discovery within the Hidden Leaf Village. Some of the elders might be worried about the potential for misuse and try to restrict Naruto's access to it.
One idea could be a crossover where Naruto characters get their hands on chainsaws in a strange, alternate universe. Maybe Naruto uses a chainsaw - like weapon as an extreme version of his Rasengan, spinning it around for a super - powerful attack.
One idea could be a fanfiction where Denji gets transported to a modern-day city outside of his own world. He has to adapt to new technology and deal with a different kind of devil threat. Another could be a story that focuses on Power's past before she met Denji, exploring her time in Hell and how she became the way she is. A third idea might be a fanfiction where Makima is actually a double agent, secretly working against the devils she seems to be controlling.
One idea could be creating an OC who is a devil hunter from a different country with unique fighting techniques. For example, an OC from a martial - arts - rich nation like Thailand, who uses Muay Thai moves along with devil - powered weapons. Another idea is an OC that is half - devil and half - human, struggling with their identity and trying to find their place in the world of Chainsaw Man. They could have powers that are a blend of both devil and human capabilities, making their story full of internal conflict.
One idea could be that Naruto is transported to the world of Lollipop Chainsaw. He might use his ninja skills to fight the zombies there. His shadow clone jutsu could be really useful in confusing the zombies and helping Juliet Starling, the main character of Lollipop Chainsaw.
A fanfic could revolve around a tournament. Characters from Naruto and Lollipop Chainsaw are forced to participate in a deadly tournament. Naruto and Juliet form an alliance early on. They face off against various opponents, some with unique jutsu - like abilities from Naruto's world and some with the grotesque powers of the zombies from Juliet's world. Along the way, they also learn about each other's cultures and values.
One interesting idea could be the combination of Naruto's ninja skills with mecha technology. For example, Naruto might use his chakra to power up the mecha in unique ways. Another idea could be the new enemies that are created specifically for this mecha - Naruto concept, like robotic shinobi that require different combat strategies compared to normal foes. Also, the design of the mecha Naruto itself can be really cool, perhaps with features inspired by the Nine - Tails.
Another idea is that Aizen somehow ends up in the Naruto universe during the time of the Fourth Great Ninja War. He could try to manipulate both sides for his own gain. But Naruto, with his ability to see the good in people, might try to reason with Aizen and show him a different path. It could be a story of redemption or a great battle if Aizen refuses.
In a 'Naruto Mermaid Fanfiction', the mermaids could be in danger from an evil organization that wants to capture them for their precious scales. Naruto teams up with a mermaid princess to stop this threat and in the process, they learn about each other's worlds.
One idea could be a crossover where Naruto and Pucca end up in the same world due to a strange ninja jutsu gone wrong. Naruto is confused by Pucca's constant chasing of Garu, and he tries to help Garu escape in his own Naruto - style way.
Well, you could write a fanfiction where the Nine - Tails in Naruto has some sort of connection to a powerful entity in the Seraphim universe. This connection could lead to a series of events that require Naruto and the Seraphim characters to work together. Or perhaps a character from Seraphim is reincarnated as a ninja in the Naruto world and has to adapt to the new way of life while still having some of their Seraphim abilities.