A simple plot could be that Shirou is transported to the Highschool DxD world and he starts hearing the 'Kyrie Eleison' chant everywhere. He is intrigued and starts to research about it. He soon finds out that it has a connection to a hidden power in this world. As he delves deeper, he has to face challenges from different factions who either want to stop him from uncovering the power or want to use him to get to it.
In the fanfiction, Shirou might meet a character in Highschool DxD who is also familiar with 'Kyrie Eleison'. They could bond over this shared knowledge. It could be a priest - like character who has been secretly observing the events in the DxD world. Together, they could try to use the power of 'Kyrie Eleison' to bring some sort of balance or peace to the often chaotic world of Highschool DxD.
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.
The plot could be that Shirou is summoned to the Highschool DxD world by accident. He discovers that his connection to Excalibur is stronger here. He is then forced to participate in the power struggles between the different factions. He uses Excalibur to protect the innocent students in the school from the more aggressive supernatural elements. Along the way, he might fall in love with one of the female characters in the Highschool DxD world, adding a romantic sub - plot to the story.
One possible fanfiction could involve Shirou being transferred to the Highschool DxD world and somehow obtaining Excalibur. Maybe he has to use it to defend the school from a new threat that the regular characters can't handle on their own. This would lead to some interesting interactions between Shirou's abilities and the magic and powers in the Highschool DxD universe.
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.
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 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.
A possible plot could be Venelana going on a solo mission to uncover a mystery that has been plaguing the underworld. This mission could lead her to discover new allies and enemies, and along the way, she could learn more about her own strength and capabilities.
A fanfiction idea could be that Lucifer from the Highschool DxD universe somehow ends up in the Naruto world. In the Naruto world, he could be seen as a powerful and mysterious figure. He might interact with the ninja clans and shinobi, and his powers could be both feared and respected. He could also have some kind of connection with the tailed beasts or the Sage of Six Paths. The story could revolve around how his presence disrupts or changes the balance of power in the Naruto world.