Asta might become friends with Rias. Rias is a kind - hearted devil and would be intrigued by Asta's anti - magic abilities. She could see his potential and offer him a place in her peerage, which would lead to a very interesting dynamic where Asta has to balance his own values with the devil's way of life. They could also work together to protect the school from various threats.
He could form a friendly rivalry with Issei. Since Issei is all about power and his Boosted Gear, and Asta is super determined with his anti - magic sword, they could compete in terms of strength and growth. They might also learn from each other's combat styles.
Asta could have an interesting relationship with the angels in the High School DxD world. At first, they might be suspicious of him because of his anti - magic, which is something that goes against their magic - based existence. But as they learn more about Asta's pure heart and his goal to protect the weak, they could come to respect him and even form an alliance with him in times of need.
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.
He could form a mentor - like relationship with some of the younger characters in High School DxD. His experience and power could help them grow. Also, he might have an uneasy alliance with the devils, as they could be interested in his power. Maybe a friendly rivalry with some of the more powerful angels.
Naruto would form diverse relationships. He has a very friendly and determined personality. So, he could bond with Issei, the main character in Highschool DxD. They could share their experiences of growing in power and facing challenges. With the female characters in DxD, Naruto would probably be respectful and friendly. He might form a brother - like relationship with some of the male devils, especially those who value strength and loyalty. However, his relationship with the more traditional and strict angels might be a bit rocky at first, as they might view his powers as something not in line with their beliefs, but through mutual understanding, they could become comrades.
There could be a storyline where Naruto is summoned to the High School DxD world by some ancient magic. He is seen as a powerful ally or a threat depending on who you ask. Rias takes it upon herself to figure out Naruto's intentions. As she gets to know him, she realizes that his values of friendship and protecting the weak are similar to her own. Together, they start a journey to stop a dark force that is rising in the DxD world, with Naruto teaching the devils some ninja techniques along the way.
It could involve Naruto's overpowered abilities in the High School DxD universe. Maybe he uses his ninja skills to fight the devils and angels there. For example, his shadow clone jutsu could be used in battles, creating multiple clones to confuse the enemies.
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.
Naruto might also interact with the Juubi on a more strategic level. He could plan with the characters from High School DxD to contain the Juubi first. For example, using the barriers and spells available in the DxD world along with his own shadow clone jutsu to surround the Juubi. Then, they could work on finding a more permanent solution, like using some ancient magic from the DxD universe combined with Naruto's sealing jutsu.
He could form a rival - like relationship with Sasuke. Both are highly skilled and have a bit of a dark side. They might constantly compete to prove who's better in combat.
In a Naruto, Ghost Rider, High School DxD fanfiction, a great storyline could be that the High School DxD world is in danger of being destroyed by an ancient evil. Naruto and Ghost Rider are brought in as the last hope. Naruto uses his friendship - building skills to gather allies among the students and factions, while Ghost Rider uses his power to fight off the minions of the ancient evil. As they progress, they discover that there are secrets in this world related to their own powers, and they must unlock them to save the day.
He could be a mentor figure. Given his vast experience and moral compass, he could guide the main characters in High School DxD. He might teach them about true heroism and the importance of using their powers responsibly.