One interesting plot idea could be that Kiba and Tsubaki have to team up for a special mission in the Underworld. They face various challenges and in the process, they discover new things about each other's pasts and their own powers. This could lead to a deeper bond between them.
A plot where Kiba and Tsubaki are chosen to represent their respective groups in a tournament. They train together and during this time, they start to have feelings for each other. However, they have to keep their emotions in check as the tournament is very important and they don't want to let their teams down. As the tournament progresses, their relationship also develops in the background.
Maybe a plot where Tsubaki gets cursed and Kiba has to find a way to break the curse. He goes on a long journey, seeking out ancient knowledge and artifacts. Along the way, they have heart - to - heart conversations and Tsubaki starts to see Kiba in a new light, and their relationship evolves from just comrades to something more special.
Another way to develop their characters is to put them in a situation where they have to make a difficult choice. For instance, they might have to choose between loyalty to their friends or following their hearts. This would force them to grow as individuals and as a pair. Their actions during this time would reveal more about their personalities and values.
One common theme is character development. Writers often explore Tsubaki's growth, her relationships with other characters, and how she evolves within the Highschool DxD universe. For example, her relationship with the main character or her progress in her own powers.
One idea could be Kiba getting involved in a secret magical tournament within the school. Another could be him forming a special alliance with an unexpected character to solve a mystery. Maybe he discovers a new power that changes his role in the school's supernatural hierarchy.
A plot could involve the main characters having to form an alliance with Trihexa to face an even greater threat. They would have to find a way to communicate and work with it, which would be a huge challenge considering its nature. This could lead to some interesting character development, especially for Issei as he tries to understand and tame Trihexa.
Well, a plot could revolve around a mystery that involves both worlds. Maybe there is an ancient artifact that has the power to disrupt the balance between the Naruto universe and the Highschool DxD universe. Characters from both sides have to work together to find and protect this artifact. This would lead to a lot of character development as they learn about each other's cultures and values while on this quest.
One idea could be a new character joining the school who has a unique supernatural ability that no one has seen before. This could disrupt the existing power dynamics and create all sorts of interesting interactions and challenges.
A plot could also focus on a new character who is a hybrid of the two universes. They have the powers of both a devil from High School DxD and a wizard from Harry Potter. They struggle to find their place in either world as they are too different from the norm in both, and they embark on a journey to create their own identity and purpose.
A plot could involve a nekoshou trying to fit into the high school environment while hiding their true nature from most students. But then they get involved in a magical tournament where they have to use their nekoshou abilities to win and prove themselves. Or perhaps a nekoshou is on a quest to find a lost nekoshou artifact that has a connection to their family's past in the Highschool DxD universe.
One idea could be Sora being a new exchange student at the school in the Highschool DxD universe. He could bring with him unique powers or knowledge that intrigue the main characters. For example, he might have a special type of magic that is different from what is commonly known in their world.
One idea could be an alien race coming to Kuoh Academy with unique powers that the devils, angels and fallen angels have to deal with. For example, an alien race that can manipulate time, causing chaos in battles and daily life.