One possible plot could be that the characters from High School DxD end up in the Bleach universe. For example, Issei could be mistaken for a substitute Soul Reaper due to his high spiritual energy. He might then team up with Ichigo to fight against a new threat that is causing chaos in both the Soul Society and the human world. The devils from High School DxD could bring their unique powers to the table, like the ability to use magic and their different forms, which would add a new dynamic to the battles in the Bleach world.
Ichigo and Issei would be a very interesting pairing. Ichigo is a powerful Soul Reaper with a strong sense of justice, while Issei is a perverted but determined devil. Their personalities are so different, yet they both have a drive to protect. They could learn a great deal from each other. Ichigo could teach Issei about self - control and discipline, while Issei could introduce Ichigo to a more carefree way of looking at things.
In a Naruto and High School DxD fanfiction, there could be a scenario where the protagonists of both stories meet in a neutral dimension. Issei from High School DxD might be intrigued by Naruto's ninja skills and determination. Naruto, in turn, could be curious about Issei's Sacred Gear. They could start training together, sharing their knowledge and skills, and ultimately have to face a common enemy that threatens both their worlds.
Asta could be brought into the High School DxD fanfic as a result of a multiverse - related event. In this case, different universes are colliding or being connected in some way. His presence would disrupt the power balance as his anti - magic is something that no one in the High School DxD world has really encountered before. He could end up training with the devils to learn more about their magic while also teaching them about the concept of anti - magic.
One way could be through the power system. The EMS (Eternal Mangekyou Sharingan) from Naruto could be a unique power that a character in the High School DxD world acquires. Maybe a transfer student from a hidden ninja village arrives at the high school and has to keep their EMS a secret while dealing with the devils and angels in the school. This could lead to some interesting battles and interactions as the powers of the EMS, like the ability to use powerful jutsus, contrast with the magic and abilities of the High School DxD characters.
One possible fanfiction could be about Naruto characters attending the school in High School DxD. For example, Naruto's positive and determined attitude might clash with the more complex social and power dynamics in the DxD world. He could make new friends or even rivals among the devils, angels, and fallen angels.