A human with a special power that makes them a killer could also be a good choice. They could be an outcast in the human world because of their power, and then get drawn into the High School DxD world. Their journey from being a feared killer to finding a place among the supernatural beings would be an exciting story to tell.
It could be Issei. Since he's always eager to become stronger and protect his friends, his encounter with the Annihilation Maker could lead to some really interesting storylines. He might be drawn to the Annihilation Maker's power and seek to learn from them to gain an edge in battles against the various threats in the High School DxD world.
Rias could also be the main character in such a story. Imagine if she goes through some sort of magical accident that makes her gain weight. As a high - status devil, this would be a huge blow to her self - image. She might become a slob as she struggles to come to terms with her new body, and it could be interesting to see how she tries to regain her confidence and status within the school and the devil society.
It could be a powerful devil who sees the Ghost Rider as a threat to his plans in the school. Maybe this devil wants to take over the school or gain more power among the students, and the Ghost Rider's presence and his sense of justice stand in the way.
A rogue devil lord who has his own twisted sense of justice. He sees the current state of the devil world in Highschool DxD as weak and wants to reshape it according to his own ideals. The killer then becomes an obstacle in his path as he tries to carry out his plans, which often involve sacrificing the innocent. This antagonist would bring in a moral dilemma for the characters.
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.
Issei would be a great character to focus on. He is the main character in the original series, and in the End Game fanfiction, there could be a deeper exploration of his growth. For instance, how he copes with the increasing responsibilities and power. His relationships with the other characters, especially his harem, could also be further developed, like his evolving feelings for Rias and how he manages his relationships with the other girls in a more complex End Game scenario.
It could be Issei. Maybe he becomes jealous of Natsu's power or popularity in DxD High School and decides to betray him for his own gain.
He might also interact with Rias. Rias is a powerful devil and a leader. Sans could approach her with his laid - back attitude. She might be initially suspicious of him but later come to respect his abilities. They could have some strategic discussions about dealing with common enemies in the High School DxD world. And Sans could probably crack some jokes that would either amuse or baffle Rias.
Asta could be brought into the High School DxD fanfic as a result of a multiverse - related event. In this case, different universes are colliding or being connected in some way. His presence would disrupt the power balance as his anti - magic is something that no one in the High School DxD world has really encountered before. He could end up training with the devils to learn more about their magic while also teaching them about the concept of anti - magic.
Well, in many High School DxD fanfictions, the Juggernaut can be a game - changer. If we consider its physical strength, it's usually off the charts. It can change the power dynamics within the story. Sometimes, the author might use the Juggernaut to create unexpected alliances. Maybe the protagonist has to team up with the Juggernaut to face a common enemy. This kind of plot twist makes the fanfiction more engaging and keeps the readers on their toes.