One possible scenario in this fanfiction could be that Harry Potter and Sasuke are somehow transported to a shared magical realm. When they encounter a Boggart, Harry might see Voldemort at first, given his past traumas. Sasuke, on the other hand, could see Itachi. But as they realize they are in this together, they combine their magical and ninja skills to defeat the Boggart. Maybe Harry uses a Patronus charm while Sasuke uses his Sharingan to disorient the Boggart.
One interesting 'Harry Potter Boggart' fanfiction could be one where Neville's boggart turns into something unexpected, like a happy Snape instead of his usual grandmother. It adds a unique twist to the character's fear and development.
Some 'Harry Potter Boggart' fanfics focus on how different Hogwarts houses might face boggarts. In Gryffindor, a common boggart could be seen as cowardice, but in one fanfic, a Gryffindor's boggart is actually the harm coming to their friends because they value friendship so much more than their own bravery.
A common boggart scenario in fanfiction is related to the characters' insecurities. For Harry, it could be that the boggart shows him as the cause of all his friends' misfortunes, emphasizing his fear of being a burden or a danger to those he loves. This can add more depth to his character and explore his self - doubts further.
In a story where Harry is raised by a boggart, he could have a very distorted view of the world. The boggart, not being a normal caregiver, might not be able to provide the love and stability that a normal family would. So, Harry might be more withdrawn and distrustful. He might also have a hard time making friends at Hogwarts as his upbringing by a boggart could have made him seem strange to other students.
In 'Harry Potter Boggart' fanfics, character development often starts with the revelation of the boggart form. If a character's boggart is public humiliation, it shows their insecurities. As the story progresses, they may face the boggart multiple times and gradually overcome that fear, which is a sign of growth.
Perhaps in the fanfiction, there is a magical tournament where both Harry and Sasuke are participants. One of the challenges involves facing a Boggart. Harry's fear of losing his wand (which is like his life in the wizarding world) and Sasuke's fear of losing his eyesight (since his Sharingan is so crucial) are what the Boggart targets. They initially struggle as they are used to fighting their battles alone. But as the Boggart gets stronger, they realize they need to work together. Harry uses spells to distract the Boggart while Sasuke uses his ninja speed to get behind it and deliver a final blow.
His magic might be different. Since he was raised by a boggart, he could have developed some unique ways of using magic related to fear or illusions. For instance, he might be able to sense when someone is about to be afraid and use that to his advantage in a duel. Also, his personality could be more solitary as he might not have had the normal social interactions growing up. He'd be more used to dealing with his own fears alone rather than sharing with others like a normal child would.
It could be a story full of surprises. The Boggart might take on forms related to the FMA world, like the homunculi, which would be really strange for the Harry Potter characters. And for EDS, Edward might have to figure out how to use his alchemy without drawing too much attention in the wizarding world. He could end up using it in a very discreet way to defeat the Boggart. For example, he might transmute a small object in his pocket into a tool to scare the Boggart away.
Often, they expand on the shape - shifting nature. For instance, a fanfic could have a Boggart that changes forms based on the collective fear of a group rather than an individual. So in a group of students, it might become a giant troll attacking the whole Hogwarts.