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.
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.
There could be. Fanfics often explore different aspects of the original story's characters and relationships. Since Issei is a central character in 'High School DxD', his family members, including his mother, could be the subject of fan - created stories. But you'd have to search through fanfic platforms to find them.
The fanfiction could also explore how the values from Naruto's world, such as perseverance and friendship, play out in the often morally - grey world of High School DxD. The orphan might try to bring a more positive and Naruto - esque attitude to the relationships between the different species in High School DxD. They could form unlikely alliances with devils, angels and fallen angels based on their own understanding of loyalty and comradeship from Naruto's story.
The reason might be to explore different directions and expand the story's universe. Maybe they got some creative inspiration and decided to implement it in Season 4 to give fans a unique experience.
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.