One idea could be a Sekirei crossover with Naruto. Picture the Sekirei in the world of ninjas, having to use their powers in new and different ways. Another could be a crossover with Bleach, where the Sekirei interact with Soul Reapers. Maybe they could help in battles against Hollows or have their own spiritual powers explored in the context of the Bleach universe.
First, you need to decide which two universes you want to cross over. For example, if it's Sekirei and Harry Potter, research both universes thoroughly. Know the rules, the characters, and the settings. Then, think of a starting point. It could be a Sekirei suddenly appearing in Hogwarts or a Hogwarts student ending up in the Sekirei world. After that, start creating your main character or characters and how they will interact with the new environment.
A Naruto and Sekirei fanfiction crossover can be really interesting. In such a story, the different power systems from both Naruto and Sekirei universes could be combined in unique ways. For example, the ninjutsu from Naruto might interact with the Sekirei's special abilities. Characters from both series could also form unexpected alliances or rivalries. Maybe Naruto characters could learn about the Sekirei Plan and get involved, or Sekirei could be introduced to the concept of chakra.
I'm not sure which is the absolute best, but 'Naruto's Sekirei Adventure' could be a great read. It could potentially combine the power systems of both Naruto and Sekirei. Naruto's chakra and the Sekirei's powers might interact in unexpected ways, like Naruto using his chakra to enhance a Sekirei's abilities or vice versa.
Typically, one main plot element is the introduction of Naruto into the Sekirei world. He has to adapt to the new rules and powers there. Another element is the interaction between Naruto and the Sekirei characters. They might form alliances or rivalries based on their different abilities and goals.
In a Naruto and Sekirei crossover fanfiction, one key element could be the power systems. Naruto has its chakra - based ninjutsu, while Sekirei has the unique Sekirei powers. Combining these would be really interesting. For example, a Sekirei character might learn to use chakra to enhance their existing powers or a Naruto character could gain some Sekirei - like abilities. Another element is the characters' personalities. The determined and often brash Naruto characters mixing with the more diverse and complex Sekirei personalities can lead to great character interactions and storylines. There could also be a new world - building aspect, like how the two very different worlds of Naruto and Sekirei merge or co - exist.
One common theme is the clash of two different worlds. Shirou comes from the Fate universe with its own set of rules, magic, and values, while the Sekirei world has its unique system of Sekirei, Ashikabi, and battles. Another theme is adaptation. Shirou has to adapt to the new rules and environment of the Sekirei world. For example, understanding how to form bonds with Sekirei as an Ashikabi.
Well, it could be about Shirou getting involved in the Sekirei battles. Maybe he uses his skills from his original story to help or compete in the Sekirei world. For example, his sword - fighting abilities might give him an edge in combat against other Sekirei or their Ashikabi.
A Sekirei and One Piece crossover would be cool too. Picture the Sekirei on the Grand Line. Their unique abilities would be a great addition to the already diverse world of pirates. They could join pirate crews as new members or cause chaos as they search for their Ashikabi among the pirates.
A common theme is power combination. Writers often explore how Naruto's powers, like his Rasengan or Shadow Clone Jutsu, can be combined with the Sekirei's powers. Another theme is the clash of cultures, as the ninja world of Naruto and the modern - yet unique - world of Sekirei collide.