She could fit in by becoming a teacher or mentor figure. Given her experience and skills, she could teach the characters in Highschool DxD about combat and honor. For instance, she could train Issei to be a better fighter not just in terms of strength but also in terms of discipline. This would also create some interesting character dynamics as the more carefree characters in Highschool DxD interact with Saber's more serious and stoic personality.
Saber could fit in as a powerful ally. Her noble phantasms and combat abilities would make her a force to be reckoned with. In a world full of supernatural beings like Highschool DxD, she would be another strong addition. She might be seen as a sort of mysterious figure that both the devils and the angels would be curious about.
Saber could be a catalyst for change in the Highschool DxD universe. Her arrival might bring new concepts like the idea of a true king or a different form of magic. This could inspire some of the characters to strive for more noble goals. Also, her code of conduct could influence the way the devils and other beings in Highschool DxD operate. For example, she might encourage them to be more honorable in their battles and dealings with each other.
Goku would fit in as a super - powerful outsider. His physical strength and combat skills are far beyond what the normal characters in Highschool DxD are used to. He could become a sort of protector for the school, using his powers to fend off any threats that come their way. His energy - sensing ability would give him an edge in detecting hidden enemies.
Well, Naruto has a wide range of powerful abilities like his shadow clones, rasengan, and chakra control. In the Highschool DxD universe which is filled with devils, angels, and fallen angels, his powers could be quite unique. His shadow clones could be used for various strategic purposes, like scouting or overwhelming opponents in battles. His rasengan, being a powerful concentrated attack, could be a real threat to the supernatural beings there. And his chakra control might give him an edge in adapting to different situations compared to the more traditional magic - based powers in DxD.
He would fit in as a unique and powerful character. His abilities would make him a force to be reckoned with. He could use his speed and strength to compete with the devils and other supernatural beings in the DxD world.
He could fit in as a super - powerful outsider. His Juubi powers would make him stand out, but his Naruto - like personality of being friendly and determined would help him make friends. He might use his powers to help the school during times of crisis, just like he did in his own world.
Naruto's abilities would bring a whole new dynamic to the Highschool DxD universe. His ability to use chakra to walk on walls and water could give him an advantage in certain terrains or situations. In battles, his ability to sense chakra could help him detect hidden enemies. Also, his summoning jutsu could call upon powerful creatures from the Naruto universe. If these creatures were introduced into the Highschool DxD world, they could change the balance of power. And his various transformation jutsu could be used for disguise or to gain temporary boosts in power.
Sasuke can fit in as a powerful outsider. His skills and abilities make him a force to be reckoned with. For example, his Sharingan can give him an edge in combat and in detecting hidden motives of characters in the DxD world.
He could fit as a powerful outsider. His power level is extremely high with Mugetsu, so he'd be a force to be reckoned with. Maybe he'd be seen as a wildcard by the different groups in DxD.
The OC, as the Son of Sparda, could fit in by being a new and powerful entity. He might be initially seen as an outsider, but his powers would quickly draw attention. He could become involved in the power struggles between the different factions in Highschool DxD. For example, the devils might try to recruit him, while the angels might be wary of his presence. His unique abilities would also make him a valuable ally or a fearsome opponent. And in the high school setting, he could be a bit of an enigma to the other students, both human and supernatural.
A plot could revolve around a spell gone wrong that merges the two worlds partially. Saber finds herself in a strange new world filled with devils, angels, and fallen angels. She might initially be confused but quickly adapt. Her presence could disrupt the balance of power in Highschool DxD. For example, the devils might see her as a threat or an opportunity to gain more power. And Saber would have to navigate this new world while trying to find a way back to her own world.
He could fit in as a new type of supernatural being. Since he has god - like powers, he might be seen as a threat at first. But then, as the story progresses, he could form alliances. For example, he could team up with the devils against a common enemy. His abilities like instant teleportation (using techniques like Flying Thunder God) would be very useful in the battles in the Highschool DxD universe.