A plot idea could be that the Time - Turner in Harry Potter is actually an ancient magical version of the Flux Capacitor. Hermione discovers this connection while researching in the library. She then tries to build a device that can combine the two technologies. This leads to her accidentally traveling to the Back to the Future universe. There, she meets Marty McFly and they have to work together to fix the time rift that her experiment created.
Perhaps a spell gone wrong in the Harry Potter universe could open a portal to the Back to the Future universe. Hermione, being the brilliant witch she is, could team up with Marty McFly. They would have to learn about each other's worlds quickly. Hermione could teach Marty about magic, and Marty could show Hermione how to use the flux capacitor. Together, they could face challenges in both universes, like stopping Biff from taking over Hill Valley in the Back to the Future world and defeating Death Eaters in the Harry Potter world.
He could meet his future children, if he has any. They would likely be powerful wizards or witches with their own adventures. Also, he might run into the future versions of Hermione and Ron. They would probably be in important positions in the wizarding world, like Hermione might be a headmistress at Hogwarts or something.
He might meet the future versions of his friends like Hermione and Ron. Their children or grandchildren could be there, and they might have different personalities and magical skills compared to their predecessors.
He'd first have to get used to the new technology. If there are new magical devices, he'd need to learn how to use them. For instance, if there are advanced wands with extra functions, it would take him some time to figure them out.
He might be shocked. In fanfiction, the future could be full of strange twists. For example, if his friends' children are involved in some wild adventures that are very different from what he expects. He would probably be confused at first, then curious to understand how things got to that point.
Well, if Harry goes back in time, his actions could have a huge impact on the future. For example, if he saves a character who was supposed to die, it could change the entire story. Maybe he saves Cedric Diggory in the Triwizard Tournament earlier than he should have. This could lead to Cedric having a major role in the fight against Voldemort later. Also, if he changes the relationships between characters in the past, like making Snape and Lily closer friends, it could change Snape's attitude towards Harry in the future.
Harry would rely on his basic values. His loyalty and courage would still be his strong suits. He might join a group of young wizards who are working on some noble cause in the future magic world. Through interacting with them, he'd gradually understand the new rules and customs. For example, if there are new dueling regulations, he'd learn them in the process of joining friendly duels.
In this fanfiction, Harry might see a world where magic has evolved in unexpected ways. For example, there could be new spells that are far more powerful and complex than the ones he knew. He could also find that Hogwarts has changed, perhaps with new buildings or different teaching methods. And maybe he would encounter descendants of his friends, who have their own unique stories to tell.
There's also 'Harry Potter and the Future's Secrets'. Here, Harry starts seeing the future after a strange encounter in the Forbidden Forest. The story focuses on how he uses these future - seeing moments to protect his friends and outwit his enemies. It has some great plot twists and really delves into the relationships between the characters in the face of this new power.