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.
In such a fanfiction scenario, Harry could use a Time - Turner. He'd have to be extremely careful not to disrupt the timeline too much. First, he'd need to figure out the exact moment in time when he could prevent Sirius from getting into the dangerous situation. Maybe he'd have to go back to before Sirius was framed for the murders. Then he could expose the real culprits and keep Sirius out of Azkaban.
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.
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.
Well, they could meet younger versions of themselves and their friends. It would be really interesting to see how they interact. They might also try to give some hints or advice to their younger selves, but they have to be careful not to disrupt the timeline too much. And they could discover secrets that were long forgotten in the present time.
In some fanfictions, time - traveling magic might work through a powerful magical artifact. For example, a special Time - Turner that is more advanced than the one Hermione used. It could have the ability to transport the user to any point in time they desire. Another way could be through a spell that requires a great deal of magical power and concentration. The caster would have to visualize the exact time and place they want to go to.
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.
He could meet new professors. These professors would have their own teaching styles and areas of expertise. They might introduce Harry to different aspects of American magic that he has never encountered before.
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.