Well, there could be a plot where the Sacred Gears in Highschool DxD are somehow connected to the ninja powers in Itachi's world. Itachi discovers that some of the students with Sacred Gears have a latent ninja ability within them. He then decides to train these students to help them control and enhance their powers. In return, the students of Highschool DxD help Itachi understand the complex social and power dynamics of their world, which is very different from his own.
Itachi and Rias would be really interesting. Rias is used to being in control and having power, but Itachi's calm and collected nature along with his overwhelming power would put her in a different position. She would likely be intrigued by his Sharingan and his combat abilities. Itachi, on the other hand, would be dealing with a different kind of power structure and would have to navigate the social hierarchy of the devils in Highschool DxD.
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.
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.
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.
In a Sharingan Naruto Highschool DxD fanfiction, the Sharingan could also play a role in terms of illusions. Imagine a scenario where the character uses Sharingan - based illusions to deceive the enemies from both worlds. They could make a Highschool DxD devil believe they are in a completely different environment like a dangerous Naruto - style forest filled with traps. And for Naruto characters, they could be made to see illusions of their worst nightmares or false threats from the Highschool DxD world, which would make for some really intense and engaging storylines.