Definitely Hermione. Since she's Harry's closest friend and very intelligent, she would be deeply involved in any changes Harry tries to make. Also, Snape. Harry's actions in the past could completely change his relationship with Snape. And of course, Voldemort. If Harry goes back far enough, he could potentially stop Voldemort's transformation into the evil wizard he became.
Well, in this 'Harry Potter goes back in time' evil fanfiction, Harry might go back and change things to benefit Voldemort. Maybe he'd stop Dumbledore from finding out important information or interfere with the Order of the Phoenix's formation. He could also use time - travel to learn dark magic from the past and bring it to the present, causing destruction and fear among the wizarding community.
If Harry Potter goes back in time to teach fanfiction, it would be a really strange and interesting scenario. First of all, the wizarding world might not understand what fanfiction is at first. Harry would have to explain that it's a form of writing where fans create their own stories based on the original. He could inspire young witches and wizards to be more creative with the Harry Potter story itself. Maybe some of them would come up with really unique magical adventures that even J.K. Rowling hadn't thought of. Also, it could cause some chaos as the concept of fan - created stories might be a bit too modern for the old - time wizarding community.
Well, in some 'Harry Potter goes back in time' fanfics, Harry might go back to his first year at Hogwarts. He could then use his more advanced magical skills (from his future experiences) to solve the mystery of the Philosopher's Stone in a completely different way. This might involve him befriending different people earlier or even changing the course of how the challenges in the dungeon were set up.
His father's friends, the Marauders, are very likely to interact with Harry if he goes back in time. He could become a part of their group, learn their secrets, and even participate in some of their pranks. Hagrid is another character. Harry has always had a special bond with Hagrid, and going back in time, he could strengthen that bond even more. And of course, Snape. Harry could try to change their relationship from the start and understand Snape better.
Another possibility is that an evil wizard from the past, who was previously defeated, gets resurrected or empowered by Harry's time - traveling interference. This wizard could then become the main antagonist, using Harry's actions to their advantage and wreaking havoc on the wizarding world.
One idea could be that Harry goes back in time to prevent certain dark magic spells from being created. He might try to stop Voldemort's rise before it even starts by influencing the earlier events related to the Dark Lord. Another plot could be Harry going back and changing the way he interacted with certain characters like Snape, which could lead to a completely different relationship dynamic. Maybe he could also go back to save some characters who died in the original timeline, like Sirius Black.
Neville Longbottom can also be a main character in some 'Harry Potter goes back in time' fanfictions. Given his growth and importance in the original series, in the fanfic, Harry might go back and help Neville become more confident earlier. This could have a significant impact on the fight against Voldemort as Neville could then be an even more powerful ally from an earlier time.
A cool plot twist might be that Fem Harry goes back in time and discovers that her parents were involved in a secret mission that was never revealed before. This could lead to her having to deal with the consequences of uncovering that truth. Also, she could find out that Voldemort had a different origin or a hidden weakness that was lost in history. As she tries to use this knowledge to defeat him earlier, she might face opposition from some of the older wizards who don't believe her or are afraid of the changes she could bring. It could also be that she changes the way magic is taught at Hogwarts, introducing new spells or techniques from the future that create a whole new set of challenges and opportunities for the students.
He could start by gathering a group of interested witches and wizards. Then, he might share some simple examples of fanfiction from his own time. Harry could explain how fanfiction allows for different interpretations of the existing story and characters. He could also encourage them to write their own short fanfiction pieces, starting with just changing a small part of the Harry Potter story they know.