Neville could be a great main character for this kind of fanfiction. He's not as confident in potions as some others. So, an accident where he creates a potion that makes all the plants in the Hogwarts greenhouses go wild and start to overgrow and take over parts of the castle would be an interesting story. He would then have to use his knowledge of Herbology to try and control the situation.
Hermione is often a common character. She might be helping Harry with research related to the potions professor's strange behavior or new potions they are working on. For example, she could be scouring the library for information about a rare ingredient the professor has introduced.
Perhaps it could be a relative of an existing character, like Neville Longbottom's younger cousin. Neville had his own growth in Herbology, so his cousin could be a prodigy in potions. They would have grown up in the same kind of environment where magic was a bit of a struggle at first, but then they find their true calling in potions and start to shine in that area.
Ron could also be the main character. He has his own courage and loyalty. He could be the one who has an unexpected connection with the poisonous snake, like being able to communicate with it in a special way that no one else can.
One possibility is that a potions accident could cause a time - related mishap. Maybe the potion was supposed to be a simple Sleeping Draught but due to the accident, it creates a time - loop. The characters find themselves repeating the same day over and over until they figure out how to reverse the effects of the botched potion. This could be a really interesting plot device in a Harry Potter fanfic as it would allow for a lot of character development and exploration of different choices within the same day.
Ron could also take on the role of the main character. He has his own brand of courage and loyalty. He could be the one who, in a moment of desperation, unlocks the power that is Voldemort's bane. For example, he might use his family's old magic artifacts in a new and unexpected way.
Hermione could be one of the main characters. She would be very helpful with all her knowledge of spells and magic theory. She could assist Harry in planning the layout of the village and ensuring that everything is in order.
Hermione could be one. She might come to the forest to check on Harry and end up staying to study the unique flora and fauna which have magical properties. Another could be Hagrid. He loves all kinds of creatures and the forest is full of them, so he could visit Harry often and bring him supplies.
Hermione could be a main character. She might be initially skeptical about Harry singing muggle music, but then she gets interested in the cultural aspect of it and starts researching the history of the muggle music Harry likes.
There could be an original character who is a descendant of the Uchiha that somehow ends up in the wizarding world. This character could teach fem Harry Potter about the Uchiha ways. Also, Dumbledore could play a role as he always encourages Harry's exploration of different knowledge, and he might be curious about the Uchiha knowledge that Harry is so interested in.
A young wizard from a small, forgotten wizarding village who has been an Obscurial since childhood due to the village's superstitious and cruel treatment towards magic they don't understand. This character could bring a fresh perspective as they enter the larger wizarding world.