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.
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.
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.
One idea could be that Spawn somehow gets transported to the world of Highschool DxD. He could end up being a mysterious new student at the school. His powers and dark nature would contrast with the more flamboyant and magical powers in the Highschool DxD world. Maybe he'd get involved in the battles between the different factions, using his chains and hellish powers in a way that no one in that world has ever seen before.
One idea could be that Naruto characters attend the Highschool DxD academy. For example, Naruto himself could be a new student with his unique ninja skills. He might form an unlikely friendship with Issei. Another idea is a battle tournament where ninja techniques are pitted against the devilish powers in the DxD world.
Maybe in the fanfiction, the Campione characters are invited to the Highschool DxD world to participate in a special tournament. They bring their own unique skills and knowledge, and during the tournament, they form unexpected alliances with the devils, angels, and fallen angels. This would allow for a lot of character interactions and development.
One idea could be to cross it with a magical girl anime. The main character from Highschool DxD could find himself in a world full of magical girls and form a harem with them. They could fight together against common enemies while also dealing with the typical harem situations like jealousy and misunderstandings.
One idea could be that Naruto is somehow transported to the world of Highschool DxD. He could use his ninja skills and chakra in the new world, maybe even teach some of the characters there a thing or two about combat. For example, he could show Issei how to better control his power using chakra control techniques.
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.
A cool plot idea could be a tournament where characters from both Highschool DxD and Naruto participate. The ninjas would show off their jutsu and the devils would display their powers. Also, imagine if Naruto got a Sacred Gear. He could use it to enhance his Rasengan or even create a new jutsu that combines his ninja abilities with the power of the Sacred Gear. Or perhaps, the female characters from Highschool DxD like Rias could teach some of the kunoichi in Naruto about different ways of using their femininity as a power, which could lead to some interesting character development for the kunoichi.