The relationship between Harry and Snape would be completely transformed. In the original timeline, Snape had a lot of pent - up anger towards Harry because of his resemblance to James. But if time was turned back, Snape might see Harry as an opportunity to right the wrongs of the past. He could become a mentor to Harry, teaching him advanced potions and dark arts defense in a way that Dumbledore never did. This would also change Harry's perception of Snape from a bitter enemy to a trusted ally.
Perhaps in the fanfiction, when time is turned back, Hermione decides to focus on different areas of magic. Instead of being all about books and rules, she starts to explore dark magic in order to stop Voldemort before he rises. This could put her at odds with Harry and Ron. Also, if time is turned back far enough, Dumbledore could be a student again. He might make different choices that change the whole wizarding world's future, like not getting involved with Grindelwald, which would have a huge impact on the events leading to Voldemort's rise.
One possible fanfiction could be about Harry Potter accidentally stumbling upon a time - turning device in Hogwarts and ending up in the 1940s. There, he might meet the young versions of his professors, like Dumbledore, and get involved in the events that were shaping the wizarding world at that time, such as the rise of Grindelwald.
Regret is a very common theme. Since the characters are going back in time, they often think about the things they did wrong before and try to correct them.
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.
Obviously, Harry Potter himself is a central character in such fanfiction. Then there's Hermione Granger. Her intelligence could play a big role if she also travels back in time. Another popular one is Severus Snape. His complex relationship with Harry and his past make him an interesting character to include in time - traveling scenarios.
One interesting plot could be Harry going back in time to prevent the death of his parents. He might use his knowledge of future events to outwit Voldemort before he becomes too powerful. Another could be Harry traveling back to his first year at Hogwarts and changing how he interacted with certain characters like Snape, which could lead to a different dynamic between them throughout the series.
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.
One idea could be Harry going back in time to his parents' era at Hogwarts. He could try to prevent certain events that led to their deaths. Another could be him going back to the time of the founders of Hogwarts and getting involved in the creation of the houses and Hogwarts' early rules.