Well, a great plot for this fanfiction could involve time - traveling crossovers. For example, the characters from 'Welcome to demon school' could be sent back in time to an era when demons and humans had a very different relationship. They would have to navigate this new historical setting while still being true to their own natures. Or perhaps a crossover with a space - themed story, where the demon school is visited by alien - like creatures and they have to figure out how to co - exist.
One interesting plot idea could be a crossover with a magical human school. The demon students could visit the human school and have to hide their true identities while dealing with human school problems. Another idea is a crossover with a different demon realm, where the characters from 'Welcome to demon school' have to adapt to the new rules and powers in that realm.
Perhaps there could be a crossover of the chakra system in Naruto and the spiritual energy in the fox demon world. A fox demon tries to learn chakra control and combines it with their own powers. This would create a whole new set of jutsu - like abilities. For example, they could create a fire - based attack using their fox demon fire and chakra to make it even more powerful.
One idea could be a crossover between 'High School Musical' and 'The Little Mermaid'. Imagine Ariel attending a human high school on land for a semester as part of a cultural exchange program. She could join the school's musical club and struggle with fitting in while also trying to keep her mermaid identity a secret. Another plot could involve characters from 'Beauty and the Beast' in a high school setting. Belle could be the new girl in a school full of jocks and popular kids, and the Beast could be a misunderstood loner. They might be paired together for a school project and slowly start to see each other in a new light.
One idea could be Naruto characters attending a modern-day high school. For example, Naruto could be the class clown who always gets into trouble but has a heart of gold. Sasuke might be the mysterious loner that all the girls are curious about. Another idea is a school tournament where Naruto and his friends use their ninja skills in a non - lethal way, like a sports - like competition within the school grounds.
A great plot could involve a tournament in the High School DxD world that Gohan participates in. He teams up with some of the characters from that series. His different fighting style and abilities would shock everyone. There could be some sort of mystery or conspiracy behind the tournament as well, and Gohan's integrity and sense of justice would drive him to uncover the truth.
Another idea is that there's a magic spell that mixes the worlds of Naruto and High School Musical. Characters like Sakura find themselves in the middle of a high - school musical competition. She uses her medical ninjutsu knowledge to help heal an injured singer from the High School Musical side. In return, they teach her some new singing and dancing skills which she then takes back to the ninja world.
One idea could be that the Demon Slayers are brought into the world of My Hero Academia. Tanjiro could learn about quirks and try to figure out how to use his skills in this new environment. Maybe some of the demons also gain quirks which makes them even more dangerous.
A really interesting plot could be that Naruto, as a demon, is wrongly accused of a series of attacks in the ninja world. He has to prove his innocence while also dealing with the prejudice and fear that his demon form instills in others. He might have to discover a way to temporarily suppress his demon form to gain the trust of those around him.
Maybe a crossover with a magical anime. The nanbaka characters could suddenly find themselves in a world full of magic. Some of the nanbaka characters might be really good at using magic in unexpected ways, like Jyugo using his shackles in a magical battle.
A crossover with a modern - day action story could be really cool too. Spartacus and his crew could be brought to the present day through some time - travel mishap. They then have to navigate the modern world, learning about new weapons and technology. They could use their ancient fighting techniques in modern combat situations, like taking on a powerful criminal organization.
Maybe in the fanfiction, the three - tailed beast crosses over into a modern - day human world. People initially fear it, but then they discover that it has a gentle side and is actually trying to find its way back home. It could form a bond with a group of humans who help it on its journey, leading to many adventures along the way.