In this fanfiction, Harry could go back and raise himself by first ensuring his own safety. He may secretly start training his younger self in combat magic much earlier. He also tries to change his own personality a bit, making himself more confident and less self - doubting. He uses his experiences from the future to make sure he forms better relationships with his friends from the start, and that he's more prepared for the challenges that Voldemort will bring.
Perhaps Harry goes back with the intention of preventing some of the traumas he endured as a child. He could start by improving his living situation at the Dursleys'. He might use his knowledge of magic to make sure he gets enough food and a proper place to study magic in secret. And as he grows up, he imparts his future wisdom on himself, like how to better handle the politics at Hogwarts and deal with Snape's hostility.
One possible plot could be that Harry uses some powerful time - turning magic to go back in time. He then has to hide his true identity from his younger self while trying to guide and raise himself. Maybe he teaches his younger self some advanced magic that he didn't learn until later in his original life, like more complex Patronus charms or advanced defensive spells.
One possible antagonist could be the time itself. Since Harry is messing with the natural flow of time, there could be some sort of time - based magic or entity that tries to stop him. It might be a time - keeper or some ancient magic that protects the integrity of the timeline.
Well, perhaps the plot could involve Harry having to fend for himself from a very young age in the wizarding world. He might have to figure out magic on his own, without much guidance from adults. Maybe he starts exploring the secrets of Hogwarts by himself earlier than in the original story, leading to all sorts of unexpected adventures.
In this fanfiction, Tony might be stranded on an unknown planet or in a different dimension. He has to use his wits and the little knowledge he has to not only survive but also better himself. He might start with basic survival skills like finding food and shelter, and then move on to more advanced self - improvement, such as enhancing his physical and mental abilities through self - training and self - experimentation.
One possible plot could be that Harry is sent to Hell due to a powerful dark magic spell gone wrong. In Hell, he has to face his worst fears and demons, both literal and metaphorical. He might encounter lost souls who tell him secrets about the magical world. Eventually, he manages to find a way back, using his wits and some unexpected magical abilities he discovers during his ordeal.
One possible plot twist could be that when they go back in time, they accidentally change something that has a huge impact on the present. For example, they might prevent the sorting hat from putting them in their original houses. This could lead to a completely different dynamic at Hogwarts, with different friendships and rivalries forming.
Well, perhaps when Fred goes back, he discovers that the danger to Harry is much more complex than he initially thought. There could be a traitor among the people he thought were allies in the past. Also, Fred himself might get stuck in the past and has to find a way to get back while still ensuring Harry's safety. Maybe the time - traveling magic is being manipulated by a hidden enemy, and Fred has to figure out who and how to stop it to save Harry.
One possible plot twist could be that when Harry goes back, he discovers that Snape's situation is far more complex than he initially thought. Maybe Snape was involved in some secret mission that put him in danger, and Harry's interference actually endangers the entire wizarding world in unexpected ways. For example, Snape might be on the verge of uncovering a new dark magic plot that Harry's presence disrupts.
One possible plot twist could be that Snape, in his attempt to save Harry, accidentally changes the entire future of the wizarding world. For example, he might prevent Voldemort from ever rising to power in the first place, which would mean that all the relationships and events that we know from the original story would be completely different. Another plot twist could be that Snape discovers that saving Harry is not as straightforward as he thought. Maybe there are dark forces at work that are trying to stop him from changing the past, and he has to fight against them while also trying to protect Harry.
One possibility could be that Harry, with his knowledge of magic and experience in fighting dark forces, takes Tsuna under his wing in the magical world. He could teach Tsuna about different spells and how to handle magical creatures, which would be a new and exciting experience for Tsuna who is used to his own world in 'Katekyo Hitman Reborn!'.
Well, in this fanfiction, Kakashi might introduce Harry to the concept of chakra. Since Harry has magic, Kakashi could find a way to combine the two powers. Maybe Harry would learn to use chakra to enhance his spell - casting speed or accuracy. And Kakashi would also help Harry deal with his emotional traumas, like the death of his parents, in a more ninja - like, stoic way.