There are. However, the quality can vary greatly. Some stories might delve deeper into the character development of both Harry Potter characters and Emiya. They could create a whole new plotline where Emiya's past and his connection to the concept of heroism in his own universe interact with the moral and ethical challenges faced by the characters in the Harry Potter series.
Well, it's unique because it allows for a fresh take on both the Harry Potter world and the world that Emiya is from. Writers can play with the idea of how Emiya's sense of justice would fit into the often morally grey areas of the wizarding world. It also gives a chance to see how the Hogwarts students would react to someone with Emiya's abilities and background, which is different from what they're used to in their own magical society.
The 'harry potter is op shirou emiya fanfiction' might be a creative blend of two different fictional universes. Shirou Emiya is known for his combat skills and determination in the Fate series. In this fanfiction, within the Harry Potter setting, he could be OP perhaps by having his unique magecraft abilities that are not typical of the magic in the Harry Potter world. For example, his projection magic could give him an edge over the normal spells used by wizards. This might lead to interesting plotlines like him facing off against Voldemort or helping the Hogwarts students in a way that defies the normal rules of their magic. It could also be a story about how he adapts to the new magical environment while still maintaining his OP status.
I'm not entirely sure if it's a widely popular concept. Fanfiction can be quite diverse, and while there are many crossovers, this specific one might be more niche. It depends on the overlap between the 'Harry Potter' and 'Fate/stay night' (where Shirou Emiya is from) fanbases.
One possible plotline could be Shirou finding himself magically transported to the Harry Potter world. He might use his skills in magic and combat from his own world to help Harry and his friends in their battles against Voldemort. For example, Shirou's ability to project weapons could be a unique advantage in the wizarding world.
Searching for this fanfiction can be a bit of a hunt. As I mentioned before, AO3 and FanFiction.net are your best bets. But you should also be aware that sometimes, the title might be slightly different due to things like formatting or the author's own creative liberties. So, when you search, try different combinations of the words. Additionally, some smaller, more niche fanfiction communities or blogs dedicated to either the Harry Potter series or the Fate series might also host or link to this fanfiction. You can use search engines like Google and type in the title along with keywords like 'fanfiction' and the relevant series names to increase your chances of finding it.
Harry Potter's magic could be used in tandem with Emiya's archery. Harry could use a levitation charm to guide Emiya's arrows with more precision, or use a protection spell on Emiya while he is shooting. They could also combine their skills in a strategic way during battles. For example, Emiya could draw the attention of the enemy with his archery while Harry sneaks up and uses a powerful magic spell from behind.
One interesting plot could be that Emiya finds himself in the world of Harry Potter. He might use his archery skills in the magical duels at Hogwarts. For example, he could enchant his arrows with magic spells from the Potterverse, creating unique combat scenarios.
In an 'emiya meets emiya assassin fanfiction', it could start with Emiya Assassin being sent to assassinate someone that Emiya is protecting. They cross paths and a tense confrontation ensues. However, as they face off, they both sense a strange connection. Instead of immediately resuming the fight, they take a moment to talk. Emiya Assassin reveals his reasons for the assassination, which are more complex than Emiya initially thought. They then find a middle ground and decide to work together to expose the real mastermind behind the plot to kill the person Emiya was protecting, all while exploring their own identities as two versions of the same individual.
It's hard to say. Emiya has his own set of skills and powers, like hisProjection Magic which he uses to great effect. But Emiya Assassin is more stealth - oriented and has a different combat style. If it's a head - on fight in an open area, Emiya might have the advantage due to his more versatile magic. However, if it's in a shadowy, enclosed space, Emiya Assassin could easily use his stealth to get the upper hand.