One possible plot could be that Naruto and Sasuke, married, are somehow transported to the DxD world. They have to adapt to the new rules and powers there while also dealing with the reactions of the DxD characters to their relationship.
There are likely some out there. You can start by checking popular fanfiction websites like FanFiction.net or Archive of Our Own. Just search for the key terms 'Naruto Sasuke married DxD crossover' and you might find some interesting reads.
Well, it could be a really interesting exploration of their relationship in a different context. Maybe in this fanfiction, they get married to unite their clans in a new way, bringing peace to the ninja world. Their personalities, which are so different yet complementary, would create a lot of drama and sweet moments in their married life.
One common theme is probably the reconciliation of their past differences. Since they had a lot of conflict in the original story, in the fanfiction, their marriage would be a way to show that they've moved past all that. Another theme could be the exploration of a new kind of power couple in the ninja world. They are both extremely powerful, and together, they could be unstoppable.
Their powers could combine in a really cool way. Naruto's Rasengan, which is a concentrated form of chakra, could be infused with Sasuke's elemental jutsu, like Fire Style. In the DxD universe, this combined attack could be a powerful weapon against the various enemies. For example, if they face a powerful devil, they could use this super - charged attack to overpower it.
Issei would be a likely ally. He's the main character in Highschool DxD and is always looking for ways to become stronger. Sasuke's combat skills and knowledge could be very appealing to Issei. Also, Rias Gremory could be an ally. As a powerful devil and the leader of her peerage, she would recognize Sasuke's potential and value his skills in battles against common enemies.
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.
One possible fanfiction could be about Naruto characters somehow ending up in the world of High School DxD. Maybe Naruto's skills in combat and his determination would make him a formidable opponent in the battles that often occur in the High School DxD universe. He could form interesting alliances or rivalries with the characters there.
A fanfiction might explore how the power of the Excalibur in High School DxD would interact with Naruto's chakra. Since the Excalibur is a powerful holy weapon, it could have some interesting reactions when in the presence of Naruto's unique chakra, which is different from the magic in the High School DxD world. For example, it could either enhance or nullify certain abilities depending on how the author writes it.
In a Naruto - High School DxD fanfiction, the power levels would be really interesting to explore. Naruto's sage mode and tailed - beast forms are extremely powerful. In the DxD world, there are different power systems like the Sacred Gears. It would be fascinating to see how Naruto's powers interact with the powers of the DxD characters. For example, could his Rasengan be enhanced by a Sacred Gear? Or how would he fare against a devil's magic?