The Golden Trio, namely Harry, Hermione, and Ron. Harry is the Boy Who Lived, so his presence in the past might attract unwanted attention. Hermione is the one who probably has a plan for everything. She'd be researching how to get back to their own time and also how to not mess up the past too much. Ron is there to support his friends. He might be the one who stumbles into some unexpected situations because of his more laid - back nature, but he always manages to pull through with the help of his friends.
Another plotline could be that they go back in time to learn more about the founders of Hogwarts. The Golden Trio might seek out the four founders to understand the true meaning of Hogwarts and its values better. This could lead to them bringing back forgotten spells or ways of magic that were lost over time, which could be used to strengthen Hogwarts against future threats.
The main characters are Harry, Hermione, and Ron, of course. Harry is the Boy Who Lived, and his courage and determination play a big role in whatever they do in the past. Hermione with her vast knowledge of magic and history is able to figure out how to navigate the time - traveling situation. Ron provides the loyalty and humor, often being the one to break the tension in difficult situations.
One interesting plot could be that they go back in time to change a small event that has a huge impact on the future of Hogwarts. For example, preventing a certain dark wizard from getting a powerful magical item when they were a student.
They might try to change certain events. For example, they could attempt to save Sirius earlier. Maybe they'd use their knowledge of future events to outwit Voldemort's plans more effectively in that time period.
Hermione would start by studying all the new things she could find. Ron would follow Harry's lead most of the time. Harry would use his instincts to figure out who to trust in this new future.
There's a fanfiction called 'First Year Redo'. In this story, the Golden Trio goes back to their first year and decides to focus on befriending some characters they didn't get to know well before. They also use their knowledge to prevent some small, but annoying things that happened in their first year from occurring again. It's a heartwarming story that shows how much they've grown since their first year at Hogwarts.
A lesser - known but great one is 'Time - Twisted Bonds'. In this story, the time travel element is used to create unexpected alliances and conflicts between the Golden Trio and the Black Sisters. It starts off with a bang as the characters are thrust into the different time periods, and the plot unfolds in a very thrilling way.
Harry would likely be the brave and impulsive one. He might be the first to jump into action when they face challenges during their time - traveling adventures. His role could also be to use his natural leadership skills to guide the group.
One interesting concept could be how the Golden Trio - Harry, Hermione, and Ron - deal with changing the past and the potential consequences. For example, they might try to prevent certain tragedies from the original story, but in doing so, create new and unforeseen problems.