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 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.
The back story of Harry Potter begins with the rise of Lord Voldemort. He was a dark wizard who aimed to purify the wizarding world by getting rid of non - pure - blood wizards. Harry's parents, James and Lily Potter, were part of the Order of the Phoenix, a group that opposed Voldemort. Voldemort targeted the Potters when Harry was just a baby. But Lily's love protected Harry, and Voldemort's curse backfired, leaving Harry with a lightning - shaped scar on his forehead and Voldemort weakened. Harry then grew up with his muggle aunt and uncle, the Dursleys, not knowing his true heritage until he received his Hogwarts letter at the age of eleven.
Well, it could be. On one hand, it means more adventures in the beloved wizarding world. However, it also has risks. If not done well, it might disappoint die - hard fans. But overall, if J.K. Rowling maintains her high - quality writing, it'll be good for the franchise.
One common theme is self - discovery. Since Harry is sent back in time, he gets a chance to see his past self and the world in a new light, which often leads to him learning more about his true nature. For example, in some fanfics, he realizes his hidden strengths or weaknesses. Another theme is the alteration of the timeline. His presence in the past can change small or large aspects of the future he came from, and the fanfics usually explore the consequences of these changes.
One popular fanfic in this vein is 'Time's Twist for Harry's Bottom'. It explores the unique concept of Harry being sent back in time due to a magical mishap involving his bottom (in a rather comical way). Another one could be 'Back in Time, Bottom First' which has an interesting plot where Harry has to deal with the changes in the past while his bottom - being the source of the time - travel has some strange side - effects.
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.