One possible reason could be a dark magic spell gone wrong. Maybe some wizard tried to experiment with a time - altering spell and accidentally reversed whatever transformation Voldemort (Riddle) had undergone. Another reason might be related to some ancient magic artifact. If Riddle's essence was somehow bound to an object, and that object was tampered with, it could bring him back to his old self.
The wizarding families would be divided in their reactions. Some families, like the Weasleys, would be ready to fight against Riddle immediately. They would gather their resources and join forces with the Order of the Phoenix if it still exists. Other families, who might have been sympathetic to the dark arts in the past, could be more hesitant, not sure whether to side with Riddle or the rest of the wizarding world.
The self - insert could change Tom Riddle's fate by showing him true friendship. In the original story, Tom Riddle was a lonely boy who turned to dark magic. If the self - insert befriends him and is always there for him, it might make him less likely to seek power through dark means.
A good old fanfiction could be the one where Tom Riddle's relationship with the Chamber of Secrets is explored in more detail. It might show how he found out about it and his early experiments with the Basilisk. This could give a new perspective on the events in the original books.
An old Harry Potter fanfiction about Tom Riddle could be about his time as a young wizard trying to gain followers. He was a very intelligent and manipulative character. Fanfictions might explore how he used his charm and intelligence to persuade other students to his way of thinking. Maybe he starts a secret club where they study dark magic together, which is a precursor to his later Death Eaters.
Well, his return would bring back a lot of memories. Some of his friends might have moved on in different ways, so there could be some initial awkwardness. But overall, they would be glad to have him back. They might also look to him for guidance in new problems that the wizarding world is facing, since he has so much experience dealing with danger.
One reason could be that he wants to find new ways to manipulate the story. Since fanfiction can be so diverse, he might be looking for ideas that he can use to his advantage. Another reason could be a strange form of entertainment. Even though he's a dark character, he could still find the creativity in fanfiction amusing or useful in some way. And maybe, just maybe, he has a hidden love for the world of Harry Potter that he doesn't show in the original story.
Another plot could be that as Harry raises Tom, he discovers some hidden good in Tom that was overshadowed by his later actions as Voldemort. Harry might then use this discovery to try and rewrite the future, preventing all the destruction that Voldemort caused. He could encourage Tom's magical abilities in a positive way, like getting him involved in helping others with magic instead of using it for domination.
In the fanfic, when Harry meets his parallel self, they could team up. They could combine their magical skills and knowledge. For example, if one Harry has mastered a particular spell better than the other, they could teach each other. This union could make them a very powerful force against any dark magic that threatens their world.
Well, in such a fanfiction, there could be a lot of self - discovery. Maybe the old Merlin gives advice to his younger self. For example, he could tell the younger Merlin about future challenges and how to avoid some big mistakes.
One possible reason could be that in the fanfic, the author might twist the story to make it seem like his parents' actions had unforeseen consequences that affected Harry negatively. For example, their fight against Voldemort led to Harry being an orphan and having to bear a great burden from a young age.