Well, you could write a fanfiction where the reincarnation is of a character from a completely different genre, like a science - fiction world. They could bring advanced technology concepts into the High School DxD universe. This would create a whole new dynamic as they try to adapt to the magical and demon - based society while also trying to introduce their technological ideas. Or perhaps the reincarnated character could be a historical figure. They might have different values and leadership skills which could change the power dynamics among the various factions in the High School DxD world. Also, a reincarnation fanfiction could center around a character who is reincarnated as an object in the High School DxD world, like a magical sword, and they have to figure out how to regain their human form and their role in the story.
One idea could be to explore a new character reincarnating into the High School DxD world with a unique power, like the ability to control time in small bursts. This could lead to some really interesting plot twists, such as being able to freeze time during battles or rewind to correct mistakes. Another idea is to have a reincarnated character who has memories of a different type of mythology and tries to incorporate that knowledge into the existing world of High School DxD. For example, if they remember Norse mythology, they could introduce new spells or artifacts related to it. A third idea is to focus on a reincarnated character who is trying to change the fates of some of the main characters. Maybe they know how things are supposed to play out from their past life knowledge and want to make things better for their favorite characters.
A simple yet interesting idea for High School DxD Reincarnation fanfiction is to have a reincarnated character who is a huge fan of the original series in their past life. So when they are reincarnated into the world, they are super excited but also have to deal with the real - life consequences of being in such a dangerous world. They could try to use their knowledge of the series to their advantage, but often end up making things more complicated. Another thought is to have a reincarnation where the character is from a parallel universe version of the High School DxD world. They have different rules and abilities in their own world, and when they come to this one, they have to learn how to function and make a place for themselves. And of course, you could do a reincarnation story where the character has a special connection to one of the main characters, like they are reincarnated as a long - lost relative of Issei's and that changes the relationship dynamics and the overall story.
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.
A third concept could be a killer who was once a victim of the dark side of the High School DxD universe. They now seek revenge on those who wronged them. However, as the story progresses, they realize that revenge is not the answer and end up joining forces with the good guys to make the world a better place.
One idea could be Livia forming an unexpected alliance with a character like Issei. Maybe she has some unique knowledge about the supernatural world that she shares with him to help in a difficult situation.
You could do a crossover with 'Sword Art Online'. The virtual reality aspect of SAO could be combined with the supernatural elements of High School DxD. For example, the characters could be trapped in a virtual world that has elements of both worlds, and they have to fight their way out. Issei might have to use his skills in a new digital environment while also dealing with SAO - like challenges.
There could be a fanfiction where Sasuke's Sharingan has an unexpected interaction with the supernatural elements in High School DxD. For example, his ability to see through illusions could be enhanced or altered by the magic in that world. This could lead to him having a different role in the battles and relationships within the High School DxD setting.
One idea could be exploring the backstory of an exorcist character in more depth. Maybe they have a hidden past related to the devils in High School DxD that no one else knows about.
One idea could be that the reincarnated character has a unique power from their previous life that completely changes the power dynamics in the DxD world. For example, they might have the ability to control time, which is unheard of in the DxD universe. This could lead to all kinds of interesting plot twists, like being able to freeze time during a battle or go back in time to prevent certain events from happening.
A plot could involve Kaido being sent on a special mission by the Devil King to infiltrate a human school. During this, he starts to develop real friendships with the characters in High School DxD. His initial motives are questioned as he begins to care more about his new friends than his mission, creating a lot of internal conflict.
One idea could be a Dragonborn character arriving in the High School DxD world and having to adapt to the unique school and supernatural environment. They might use their dragonborn abilities to stand out in the Rating Games.
One plot idea could be that Naruto is summoned to the High School DxD world by a powerful magic user who mistakes him for a powerful demon. Once there, Naruto has to prove that he is not a threat while also trying to find a way back home. Along the way, he befriends some of the students at the school and helps them with their own problems, like protecting the school from a sudden attack by a group of rogue devils.