One idea could be Shirou using his projection magecraft in the Highschool DxD world to create unique weapons during battles against the devils or fallen angels. For example, he might project Excalibur to fend off a powerful devil attack. Another plot could involve Shirou forming an alliance with Issei and his group, teaching them about his different combat techniques and how to better strategize in fights. He could also have a romantic sub - plot with one of the female characters from Highschool DxD, like Rias, where his noble and self - sacrificing nature attracts her.
Shirou would fit in as a powerful and unique addition. His combat skills and magecraft would make him a formidable ally or opponent. He could use his projection abilities to create various weapons and tools, which would be very useful in the battles that often occur in the Highschool DxD world.
Well, Shirou could form an unlikely alliance with some of the characters in Highschool DxD. Maybe he teams up with Issei and his group. Since Shirou has a strong sense of justice, he could help them in their battles against stronger enemies. He might also teach them some of his combat techniques, like how to better utilize their own powers.
A plot could involve Shirou being transferred to the Highschool DxD school. He quickly catches the attention of Rias and her peerage. His different style of combat and his unwavering determination might lead to some friendly rivalries within the group. He could also introduce some new concepts of magic that the DxD characters have never seen before, like his ability to reinforce objects. And he may find himself in the middle of the power struggles between the different factions in this new world.
In this fanfiction, Shirou might be seen as an enigma. His calm and collected demeanor would be different from the more boisterous characters in Highschool DxD. He could potentially be a mediator between the different groups, given his own experiences with complex moral situations in the Fate series.
One possible scenario in such a fanfiction could be Shirou Emiya's unique abilities from the Fate series being introduced into the Highschool DxD universe. His projection magic might cause quite a stir among the devils, angels, and fallen angels in that world.
One idea could be an OC with a unique sacred gear that has powers no one has seen before. For example, a sacred gear that can manipulate time on a small scale, allowing the OC to freeze time briefly during battles in Highschool Dxd's universe. This would add a whole new dynamic to the battles and interactions.
His projection magic might be affected by the different magical rules in the Highschool DxD world. Maybe it becomes more powerful or has some new limitations. For example, projecting holy swords could have a different energy cost or be more effective against certain types of devils or fallen angels.
Well, there are fanfictions that explore Shirou's moral code in the context of Highschool DxD's world. For example, he might struggle with the more hedonistic and power - hungry aspects of some of the characters in DxD. He could use his skills as a hero of justice to try and bring some order or a different perspective to the battles and power plays in the DxD universe. There are also fics that focus on his relationships with the female characters in DxD, like how he would interact with Rias or Akeno, given his more reserved nature compared to the typical DxD male characters.
One plot idea could be Dark Shirou in a post - apocalyptic world where he uses his skills to survive and rule a small territory. Another could be his journey to find a way to reverse his dark transformation. Or a plot where he has to face his darker self in a psychological battle within his own mind.
A creative one could be where Shirou's reality marble from the Fate series somehow merges with the Highschool DxD world, creating a really unique and strange new setting. For example, the landscapes from his reality marble start to appear in the DxD world, and the characters have to figure out what's going on. Another creative fanfic might be Shirou being reincarnated as a devil in the DxD world but still retaining his memories and abilities from his past life, leading to some really interesting power - ups and plot twists.