Well, the plot could start with Naruto already being a delinquent in the High School DxD setting. He has a reputation for causing trouble, but it's all because he's trying to find a way back to his own world. Along the way, he meets Rias Gremory and her peerage. At first, they're suspicious of him, but as they see his skills and determination, they start to work with him. Naruto's delinquent behavior could be a cover for his loneliness and confusion in this new world, and as he bonds with the characters from High School DxD, he slowly changes and becomes more integrated into their world while still holding onto his ninja values.
A possible plot could be that Naruto is somehow transported to the world of High School DxD. He's seen as a delinquent because he doesn't follow the school's rules which are very different from what he's used to in Konoha. He might start fights with some of the devils or fallen angels in the school who underestimate him. As the story progresses, he forms alliances with some of the students and they work together to solve problems in the school, like stopping a powerful enemy that threatens both the human and supernatural students.
In a Naruto and High School DxD delinquent fanfiction, the characters from both series could form interesting relationships. Naruto's positive attitude might clash with some of the more devilish or mischievous characters in High School DxD. He could try to reform some of the delinquents there, using his own experiences of being an outcast in Konoha at first. This could lead to some really cool character development and interactions between the two universes.
Well, it could be that Naruto, as a Nephilim, enters the High School DxD world. He might use his ninja skills along with his new Nephilim powers. Maybe he has to deal with the devils and angels in the school setting, while also trying to keep his identity a secret. He could form new alliances and rivalries within the school, all while facing challenges that are unique to this crossover world.
Perhaps in this fanfiction, Naruto and Kushina are reincarnated in the High School DxD world. They have to adapt to the new school life there. Naruto would surely stand out with his determination and fighting abilities. Kushina would be a motherly figure to some of the students who lack that kind of support. They both could change the balance of power between the different factions in the world.
One possible plot could be that Naruto and Hinata are transferred to the High School DxD world. Naruto's powerful ninja skills might make him stand out, while Hinata's shy yet strong - willed nature could attract different reactions. Maybe they have to face the devils and angels in this new world together, with Naruto using his Rasengan and Hinata her Gentle Fist techniques to protect their new friends and find a way back to their own world.
One possible plot could be that Naruto, who is now half - youkai, gets transferred to the High School DxD world. He has to hide his true identity at first but then gets involved in the power struggles between the different factions. Maybe he uses his ninjutsu and new - found youkai powers to protect his friends and the school from a common enemy.
One possible plot could be that Kiba is made out to be a bully in the High School DxD setting. He could be picking on characters from the DxD world, which angers the main characters from that series. This then leads to a conflict where the DxD characters team up with some Naruto characters who also don't like Kiba's behavior.
One possible plot is that Gilgamesh views the High School DxD setting as beneath him at first. But as he gets to know the characters like Issei and Rias, he starts to see the value in their struggles. He could then use his vast resources and power to help them in their fight against stronger enemies, all while maintaining his haughty and kingly demeanor.
One possible interesting plot could be that Naruto, with his unique ninja skills, gets transferred to High School DxD's world. He might initially struggle to adapt to the different rules and magic there. For example, he could mistake a devil's magic for some kind of new jutsu. Then, he could end up teaming with Issei and his group to fight a common enemy that threatens both the ninja world and the world of High School DxD. Maybe an ancient evil that wants to use the power of both chakras and devil magic for its own dark purposes.
A great plot could be a tournament - style story in the 'Naruto x Rias High School DxD' fanfiction. The tournament could be a mix of ninja and devil skills. Naruto enters representing his ninja heritage and Rias enters on behalf of the devils. Along the way, they have to face opponents from different factions. As they progress, they start to understand each other better and develop a deeper bond. Naruto's determination to win for the sake of his new friends in the DxD world and Rias's desire to prove the strength of the devils would drive the story forward, and their combined efforts could lead to an unexpected victory.
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.