Another possibility is that Ron Weasley could be the main character in this kind of fanfiction. Ron has always been interested in Muggle things, and cars are very much a Muggle invention. He could start collecting cars and get into all sorts of adventures because of it. Maybe he would try to introduce his car - collecting hobby to Harry and Hermione, and they would have different reactions. Hermione might be skeptical at first but then get drawn into the mystery and magic associated with the cars.
One interesting storyline could be a wizard who is also a car collector. Maybe he uses magic to enhance his cars, like making them fly or be invisible. Another could be a story where Harry Potter himself discovers the world of car collecting during his travels and is fascinated by the Muggle contraptions. Or perhaps a group of Hogwarts students start a car - collecting club as a secret society within the school.
How about Neville Longbottom? He could be the main character in a fanfiction set in the Chamber. Neville, who has always been underestimated, could show his true courage and strength. He might discover a hidden ability that helps him face the challenges in the Chamber, perhaps something related to his connection with plants, which could play a crucial role in defeating the Basilisk or whatever new threat is in the Chamber.
Obviously, Harry Potter himself could be the main character. He could be the one driving the car or be a passenger when the accident occurs. His friends like Ron and Hermione would also likely be involved, either in the car with him or coming to his rescue after the accident.
A lesser - known character like Ernie Macmillan from Hufflepuff could be the main character. As a prefect, he could bring out the unique qualities of Hufflepuff house. He might have to deal with issues like other houses underestimating Hufflepuff students. His story could focus on how he promotes the values of hard work, loyalty, and fairness within the house and the school as a whole.
Hermione could be a great main character. She's curious and intelligent, so she would be really interested in exploring the secrets of the diary. She could use her vast knowledge of magic to try and figure out how the diary works and what secrets it holds.
Harry Potter himself could be the main character as a revenant. Maybe he has some unresolved issues from his past, like his relationship with his parents. His revenant form could be on a journey to fully understand their sacrifice and his own place in the wizarding world's history.
It could be a younger sibling of one of the main characters in the original series. For example, Ginny's little sister who has big shoes to fill but also a great passion for Quidditch.
Hermione could be a great main character in an elementalist fanfiction. Her intelligence and thirst for knowledge would make her eager to learn about and master elemental magic. She could be the one to discover a long - lost elemental spell book and start exploring the powers.
It could be a new student at Hogwarts who has a natural talent for dealing with beasts. This student could be from a family that has always had an affinity for magical creatures but has kept it a secret until now.
A Muggle - born mouse. It could be a unique concept where a mouse from the Muggle world somehow ends up in the wizarding world. It would be interesting to see how it copes with all the magic and the rules of the wizarding society.