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 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 the protagonist from DarkSiders ends up in the world of Highschool DxD. Maybe he has to navigate the complex social and supernatural hierarchies there while also trying to find a way back to his own world. He could bring his unique combat skills and abilities from DarkSiders and use them in the battles that often break out in the Highschool DxD universe.
One idea could be a crossover where Naruto characters are transported to the world of The Highschool of DxD. For example, Naruto himself could end up at the high school and his ninja skills would be quite a shock to the devils and other beings there. He might get involved in the Rating Games, using his shadow clones and other jutsu to his advantage.
One idea could be that Ben Tennyson somehow ends up in the world of Highschool DxD. Maybe his Omnitrix gets a strange power-up that transports him there. He could use his alien forms to interact with the characters in DxD, like using Four Arms in a fight against some of the devils or fallen angels.
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.
A fanfiction could focus on Ichigo attending the school in Highschool DxD. His presence would surely disrupt the normal balance. He might not be interested in the whole 'devil' and 'angel' thing at first, but gradually get involved. He could also introduce some of his Soul Reaper training methods to the students there, which would be very different from what they're used to.
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.
One possible fanfic could be about Ravel from Highschool DxD finding herself in the Naruto world. Maybe she could use her unique abilities, like her magic skills, to interact with the ninjas. For example, she might befriend Naruto and help him with a difficult mission using her magical powers to provide an unexpected advantage.
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.