A third idea is that there's a mix - up with the curses in both worlds. Ranma's curse of changing into a girl when splashed with cold water gets combined with some of the magic in Highschool DxD. This creates a whole new set of problems and adventures for him as he tries to figure out how to deal with this strange new situation while also trying to keep his identity a secret from the new characters he meets.
Well, it could be about Ranma getting transferred to the Highschool DxD world. Maybe he'd get involved in the supernatural battles there, using his martial arts skills. He might team up with Issei and the others, and his unique curse could add an interesting twist to the story. For example, his transformation between male and female forms could lead to some comical or strategic situations during fights.
Ranma would probably be very confused at first when he enters the Highschool DxD world. He might be skeptical of the devils and angels. Issei, being a pervert, might be really interested in Ranma's female form. But Ranma would likely not tolerate Issei's behavior easily. Kuno from Ranma 1/2 might see the DxD characters as new rivals for Ranma's affections. And the female characters from DxD could be intrigued by Ranma's martial arts skills and his tough - yet sometimes clumsy - personality.
The most interesting plot could be one where Ranma and Issei swap bodies. Issei would have to deal with Ranma's martial arts world and his curse, while Ranma would have to handle Issei's life as a perverted devil. This would lead to a lot of comical misunderstandings and also some deep character development as they both learn from each other's experiences and try to get back to their original bodies.
One idea could be that the characters from Highschool DxD end up in the Hellsing universe. Maybe Issei and his friends are mistaken for new types of vampires due to their devil powers. They then have to team up with the Hellsing organization to clear their names and also fight against a common enemy that is threatening both the devil and human worlds in this combined universe.
One possible fanfiction could be about Naruto somehow ending up in the world of Highschool DxD and meeting Moka. Maybe Naruto's skills and his never - give - up attitude would impress Moka and the other characters. He could teach them some of his ninja techniques, like the Shadow Clone Jutsu, which could be useful in battles against the devils and other supernatural beings in the Highschool DxD world.
One possible scenario in such a fanfiction could be Shirou Emiya's unique abilities from the Fate series being introduced into the Highschool DxD universe. His projection magic might cause quite a stir among the devils, angels, and fallen angels in that world.
The theme of friendship is quite common. For example, in a crossover with 'My Hero Academia', the characters from both series form new friendships and alliances. They learn from each other's strengths and weaknesses. Also, the theme of adaptation is present. Characters from Highschool DxD have to adapt to the new rules and environments of the crossover world. Say, if they cross over with a post - apocalyptic world, they need to figure out how to survive and use their powers in that new context.
One possibility could be that Ichigo's powers would make him a very unique and powerful character in the Highschool DxD universe. His zanpakuto abilities and high - level combat skills could give him an edge in the battles against the devils and other supernatural beings in that world.
There's a crossover that combines Highschool DxD with the Naruto universe. In this fanfiction, the ninjas' skills are pitted against the devils and angels of Highschool DxD. It's a fun read as it explores how the different power systems would interact. The author does a great job of keeping the characters' personalities intact while introducing new and exciting storylines.