One idea could be to have the characters from Dragon Ball Z attend the school in Highschool DxD. For example, Goku could be a transfer student with his incredible power causing quite a stir among the devils, angels and fallen angels. He might be confused by the different social dynamics and the whole concept of the supernatural school setting.
Another approach could be a story where the antagonists from both series team up. The villains from Dragon Ball Z like Frieza and the ones from Highschool DxD such as the Khaos Brigade could join forces. This would force the heroes from both universes to also work together to defeat this new and powerful threat.
One idea could be a crossover where Goku and Naruto meet. Maybe they start off as rivals, each thinking their own world's way of fighting is superior. But then they face a common enemy that forces them to combine their powers. For example, a powerful demon that can absorb chakra and ki. They would have to learn from each other's techniques to defeat it.
Plot is important too. You could start with a simple premise like a rift in the multiverse that brings the two worlds together. Then build on it with sub - plots such as a tournament where characters from both series compete. This not only allows for exciting battles but also for characters to learn from each other. Also, don't be afraid to add your own twists and turns to keep the readers engaged.
Another idea is a fusion of the power systems. Maybe in the fanfiction, the Sacred Gears from Highschool DxD could be enhanced by the Dragon Balls. The Dragon Balls' wishes could be used to unlock new forms or powers in the Sacred Gears. This would create a whole new level of power - ups for the characters in Highschool DxD.
One idea could be a crossover where the powerful Saiyans from Dragon Ball Z end up in the Sekirei universe. They might get involved in the Sekirei Plan, either as protectors or accidental disruptors. For example, Goku could befriend some of the Sekirei due to his kind nature and his love for a good fight.
One idea could be a crossover where Naruto characters end up in the world of Highschool DxD. For example, Naruto himself could be transferred to the school in Highschool DxD. He might use his ninja skills in the battles against the devils and fallen angels there. His shadow clone jutsu could really surprise the characters in the Highschool DxD world.
One idea could be a crossover where Goku meets Superman. They could have a friendly sparring match to test each other's strength. Maybe they start off misunderstanding each other's motives but then realize they are both protectors of their respective universes.
There's also 'Dragon Ball Z Meets Highschool DxD: A New Adventure'. This fanfiction explores how the characters from the two universes interact when they are suddenly thrust together. It delves into how the power levels from Dragon Ball Z compare to the abilities in Highschool DxD and has a lot of humorous moments as the characters from different backgrounds try to understand each other. It's quite long, so there's a lot to get into. The author does a great job of developing the story over time, introducing new plot twists and character developments that keep readers engaged. It's definitely a must - read for fans of both series.
You can try websites like FanFiction.net or Archive of Our Own. These platforms usually have a wide variety of fanfictions, including ones related to Dragon Ball Z and Highschool DxD. Just use their search functions to look for the specific fanfictions you're interested in.
A more complex fanfiction could be about a joint mission between the two universes. The leaders of both worlds realize that there is a common threat that only the combined forces of Highschool DxD and Naruto can defeat. So, they send their best characters, like Rias and Sasuke, to work together. This would involve a lot of character development as they learn about each other's cultures and powers.
A story could also revolve around a quest that involves both Diablo and Highschool DxD elements. The characters from both universes could team up to find a powerful artifact that has significance in both worlds. They would need to overcome their differences and work together, with the devils and angels from Highschool DxD learning about the dark and dangerous forces from Diablo.