One interesting storyline could be Alucard forming an unexpected alliance with the main characters in High School DxD. Maybe he uses his unique powers to help them in a battle against a common enemy.
A possible storyline is that Alucard has some sort of past connection with a character in High School DxD that is gradually revealed. This connection could change the dynamics between different factions in the story.
Another storyline might involve Alucard becoming a teacher at the high school in the DxD universe. His unorthodox teaching methods could lead to all kinds of chaos and character development for the students.
Issei could also be involved in a storyline where he has to make a difficult choice between different paths. Maybe he has to decide between following a more peaceful route or a more aggressive one to protect his friends and the ones he loves, which would add depth to his character in the fanfiction.
A third storyline could focus on a mystery. There are strange happenings around Sans Seraphim High School, like students disappearing without a trace. The characters from High School DxD start to investigate and uncover a dark secret that has been hidden for a long time, perhaps something related to an ancient evil that is now awakening.
One interesting storyline could be about the characters' unique power development within the fanfic's universe. For example, a character might gain new powers related to their'slave' status that gives them an edge in battles or challenges. Another could be a story of friendship and loyalty that develops between the main characters despite the complex'slave' relationships depicted in the fanfic.
Rias Gremory is also quite popular. When she is paired with Alucard in fanfiction, their different personalities and powers create interesting scenarios. She is a strong and intelligent character, and her relationship with Alucard can range from friendly cooperation to a more complex, power - based relationship.
One possible storyline could be that Naruto, with his unique skills and positive attitude, enrolls in the high school from High School DxD. He might form an unlikely friendship with some of the characters there, like Issei. Together, they could face off against common enemies, with Naruto teaching Issei some of his ninja techniques, adding a new dimension to the battles in the High School DxD universe.
One interesting high school dxd fanfiction could be those that explore different character relationships in more depth. For example, a story might focus on the friendship between Issei and Kiba, showing their adventures outside of the main plot's battles. Another could be a fanfiction where Rias has to deal with a new kind of threat that forces her to use her leadership skills in a completely different way. There are also fanfictions that give side characters more backstory, like how Akeno came to be part of Rias' peerage.
A great idea for a High School DxD fanfiction is to explore a romantic relationship that is not the typical one in the series. For instance, a story about a relationship between two side characters who have to overcome many obstacles due to their different species or social status within the devil - angel - fallen angel hierarchy. You could also write about a character's journey of self - discovery outside of the main plot of the series. Like a character who realizes they have a different purpose in life than what they were initially led to believe.
One interesting storyline is the growth of the main character in his power and relationships. He constantly faces challenges and overcomes them, forging stronger bonds with his friends and allies along the way.
Maybe a story where Alucard, Naruto, and a character from DxD end up in a strange new dimension. Naruto uses his ninja skills to scout the area, Alucard provides protection with his vampiric abilities, and the DxD character uses their own powers to try and find a way back home. Along the way, they have to face new and dangerous enemies that are a mix of all three universes.
One interesting fanfiction could be a crossover where Naruto characters end up in the High School DxD world and have to adapt to the different rules and powers there. For example, Naruto might use his chakra in unique ways against the devils and angels in the DxD universe.