A less obvious but interesting choice would be Batman. Batman's detective skills and gadgets combined with Shirou's combat abilities would make for an amazing duo. They could work together to take on Gotham's criminals and any threats that cross over from Shirou's world.
Another good choice would be Harry Potter. Imagine Shirou in the wizarding world. He could use his tracing ability to create magical items, and he could also learn new spells from the Hogwarts professors. They could team up to fight against dark wizards.
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 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 common theme is power and its consequences. Since Shirou has godlike powers in these fanfictions, it often explores how such great power affects him, whether it corrupts him or makes him a better person.
I think 'Godlike Shirou's Grand Adventure' is a really good one. This fanfiction has Shirou with godlike powers in a crossover scenario. It starts with Shirou suddenly finding himself in a world full of different characters from other franchises. He has to use his newfound powers to not only survive but also to protect those he cares about. The story delves deep into his character, exploring how his power changes him and his relationships with others. It also has some really cool action scenes that are well - written.
A great Shirou Emiya crossover fanfic is 'Shirou and the Avengers'. Imagine Shirou in the Marvel Universe. He could project Mjolnir and use Thor's power. His selfless nature would fit well with the Avengers' values. Also, 'Shirou in the Dragon Ball Universe' is fascinating. He might train with Goku and learn ki control, while also showing his own swordsmanship skills against powerful foes in that universe.
One popular Shirou Emiya crossover fanfiction could be where he crosses over with the Fate/Grand Order universe. In this, Shirou might interact with different Servants from the game in new and exciting storylines. Another could be a crossover with the Nasuverse in general, exploring his role in different settings within that vast fictional cosmos.
To develop Archer in this crossover, focus on his past experiences and how they contrast with Emiya's. For example, Archer's jaded view of heroism can be explored further when he meets Emiya, who is still somewhat idealistic. You can have them have in - depth conversations about what it means to be a hero. Emiya, on the other hand, can grow by learning from Archer's combat skills and more practical approach to magic. Maybe they get into a fight where Emiya realizes he needs to be more strategic like Archer.
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.
Well, Shirou in crossover fanfiction often shows his tenacity. No matter how strange or difficult the new world he is in, he doesn't give up easily. In a crossover with the Digimon world, he faces many challenges but keeps fighting. His loyalty is also a big part of his character. If he makes friends in the new world, he will be loyal to them and protect them at all costs. And his growth as a character is interesting. He may start with his basic set of skills from his original world, but as he experiences new things in the crossover world, he evolves, both in terms of his combat abilities and his understanding of different cultures and values.
They play with the differences in the universes. Let's say a crossover with a post - apocalyptic world. Shirou's skills and values would be tested in a whole new way. The writer has to create new challenges for him, like how he would deal with limited resources while still trying to uphold his heroic nature. Also, they might introduce new characters from the crossover world that can either help or hinder Shirou, adding more depth to the story.