A plot could involve a werewolf being sent to the 'familier of zero' universe. The werewolf's abilities would be a mystery and source of both fear and fascination. They could end up being hunted or befriended depending on how the story unfolds. It could also lead to a discovery of a common enemy between the two universes.
A werewolf alpha could be the main character. He enters the 'familier of zero' world and has to navigate the complex magic and politics there. His werewolf instincts could either help or hinder him as he tries to understand this new world and find a way back home.
One idea could be that Naruto is bitten by a werewolf during a mission in a hidden forest. He then has to struggle with his new werewolf nature while still trying to protect the village. Another plot could involve a werewolf clan that has a prophecy about Naruto and they either try to help or hinder him depending on how they interpret it. A third idea is that some of Naruto's friends also get turned into werewolves and they have to find a cure together.
Another plot could involve Ultraman Zero crossing over to a post - apocalyptic Earth. He finds a group of survivors who are fighting against mutated monsters. Zero has to use his powers not only to defeat the monsters but also to help the survivors rebuild their society, teaching them about hope and perseverance.
Perhaps in the fanfiction, Zero and Kiryu both get cursed by an ancient magic. The curse gives them some of each other's traits while also weakening their own original powers. This forces them to rely on each other more than ever. They go on a quest to find the source of the curse and reverse it. Along the way, they learn about each other's pasts, values, and what really drives them, leading to a deep friendship or respect for one another.
Another plot could involve the Pleiades from Overlord. They end up in Re:Zero and are mistaken for some sort of magical spirits. Subaru tries to enlist their help in his quest to save Emilia. The Pleiades, with their combat skills and unique abilities, could change the power dynamics in the Re:Zero world and also introduce new technologies and magic concepts from Overlord.
Maybe in the fanfiction, there's a magical or technological accident that swaps their powers for a while. Scorpion has to deal with Sub - Zero's ice powers and vice versa. This leads to a lot of comical and also challenging situations as they try to adapt to the new powers while still facing their usual foes.
Another possibility is a crossover with 'My Hero Academia'. Subaru could be a student at U.A. High School. His Return by Death power would be like a quirk. He might struggle at first to control it and fit in, but eventually use it to save his classmates from villains, in a way that surprises everyone with his growth and resourcefulness.
One idea could be to cross it with a modern - day world. Imagine Louise and Saito suddenly finding themselves in a bustling city. They would have to deal with technology they don't understand, like smartphones and cars. Another plot could involve a crossover with a magical school from another series. The characters could exchange magical knowledge and spells.
One idea could be that the characters from Fairy Tail and Edens Zero meet in a parallel universe. They initially have some misunderstandings due to their different worlds and magic systems. But then they realize they can work together to fight a common enemy, like a powerful dark mage who is trying to destroy both their universes.
Another plot idea is a time - travel crossover. Zero One could be sent back in time to the era of the first Kamen Riders. There, he has to adapt to the different fighting styles and technology of that time. He might also introduce some of his own futuristic technology to the early Kamen Riders, changing the course of their battles and the development of their powers.