One idea could be Shirou finding himself in the Rosario Vampire world and having to adapt to its unique rules. Maybe he uses his skills from his own world to protect the school from various threats that are different from what he's used to.
A possible plot could involve Shirou forming an alliance with Moka and the other characters. He could teach them some of his combat techniques, while they help him understand the supernatural aspects of their world better. For example, he might show Moka how to be more strategic in combat, and in return, she could introduce him to the different types of vampires and their powers.
Another plot could be that Shirou discovers a new, highly competitive gamer community. He starts from the bottom but his determination and strategic thinking, which he has from his heroic deeds, help him rise through the ranks. He might make new friends and rivals in this community.
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.
One plot idea could be Shirou Emiya finding himself in the world of Fairy Tail and having to adapt to the magic - based society. He might try to use his Reality Marble in unique ways to combat the dark guilds. Another could be him teaming up with Natsu and his friends to take on a powerful, otherworldly threat that has somehow entered the Fairy Tail universe. He could also learn new forms of magic from the wizards in Fairy Tail, like learning how to incorporate Fairy Tail - style elemental magic into his existing abilities.
One idea could be that Archer is transported to an alternate universe where Shirou has a completely different personality. Maybe Shirou is a villain in this world, and Archer has to figure out how to deal with him. Another plot could involve them teaming up to fight a powerful, new enemy that neither could defeat alone. For example, a powerful mage from a different dimension who wants to conquer their world.
Another plot could involve Shirou reincarnated in a modern superhero world. He might struggle to fit in at first, but then realizes his abilities can be used to fight super - villains. His noble phantasm could be adapted in a unique way to combat the new threats in this world.
One plot idea could be Shirou Emiya, as an angel, arriving at the Highschool DxD world and getting involved in the ongoing power struggles. He might use his skills from the Fate series to help out the various factions while also trying to maintain his angelic identity.
Another plot could involve Shirou getting a glimpse into the past of the Grail War through some sort of magical artifact. He discovers secrets that were long buried and this knowledge could either help him in the war or put him in greater danger as others may want to silence him.
One interesting plot idea could be that Artoria Archer and Shirou Emiya are forced to team up in a strange magical tournament. They start off distrusting each other, but as they face various powerful opponents, they gradually learn to work together and understand each other's motives better. For example, they might encounter a foe who can manipulate time, and they have to rely on Artoria's combat experience as an Archer and Shirou's determination to find a way to defeat the enemy.
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.
One idea could be that Batman ends up in the Rosario Vampire world while investigating a strange case. He has to deal with the vampires and other supernatural beings there while trying to find his way back home. Another could be a scenario where a character from Rosario Vampire travels to Gotham and causes chaos, and Batman has to figure out how to handle this new kind of threat that doesn't follow the normal criminal patterns. Maybe a vampire from Rosario Vampire becomes an ally of Batman in Gotham, and they team up to fight a greater evil that is affecting both their worlds.