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.
Akeno is another fan - favorite. Her dual nature, being both sweet and somewhat sadistic, can be explored in different ways in fanfiction. At Sans Seraphim High School, she could be involved in a love triangle or be the key to solving a mystery related to the school's dark past.
One interesting plot could be Godlike Sans using his unique abilities to help the characters in High School DxD in battles. For example, his teleportation skills might give them an edge in combat against powerful foes.
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.
One possible plot could be Sans getting involved in the supernatural battles in the High School DxD universe. Maybe he uses his unique abilities from his original game to help out the main characters in a fight against a powerful enemy. For example, his bone attacks could be a useful addition to the combat arsenal in that world.
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.
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 'seraphim sans crossover fanfiction' could involve a crossover with a space - opera setting. Imagine Seraphim Sans traveling through the galaxy, meeting different alien species and getting involved in interstellar politics. His character would add an interesting dynamic to the high - tech and often morally complex world of space - opera. He might be a key figure in a war between different star empires or help broker peace.
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.
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 common theme is power dynamics. Since 'godlike sans' is, well, godlike, it often changes the power balance in the 'High School DxD' world. Another theme could be the clash of cultures, as 'sans' comes from a very different universe compared to the one in 'High School DxD'.