Perhaps a major plot twist is that the reason he was sent to the past wasn't what he initially thought. Instead of going back to prevent a big catastrophe, he was actually sent there by a future villain who wants to change the past in their favor. And as he tries to figure out his mission in the past, he slowly discovers the true intentions behind his time - travel, which adds a whole new layer of mystery and conflict to the fanfic.
One possible plot twist could be that adult Izuku's knowledge of future events gets distorted as he travels to the past. Maybe he remembers things wrongly and ends up causing more chaos instead of preventing disasters. For example, he might think a certain villain will attack at a specific time, but in the past, the villain's plans were different. Another plot twist could be his relationships with the past versions of his friends. They might not trust him immediately, thinking he's some sort of impostor or has ulterior motives. This could lead to some interesting character dynamics as he tries to prove himself and gain their trust.
One interesting plot point could be Izuku using his knowledge from the future to change certain events in the past. For example, he might prevent some villains from rising to power earlier. Another could be his interactions with his younger self, guiding and mentoring him in a way that speeds up his growth as a hero. He could also have an impact on the relationships between other characters, like All Might and his predecessors.
Well, it could be about Sakura going back in time and trying to change certain events that happened in her life. Maybe she wants to fix a mistake or save someone she lost.
His character development would likely be more complex. As an adult, he has already experienced a lot. So when he goes back to the past, he might have a different perspective on things. For instance, he may be more patient with his younger self's mistakes compared to how he was when he was actually that age. He could also be more confident in his abilities, which would change how he approaches challenges in the past.
Obviously, he would interact with his younger self. His younger self is a key character as Izuku from the future tries to guide and influence him. Also, All Might is likely to be an important character he interacts with. All Might has a huge influence on Izuku in the original story, and in the past, Izuku could have a different kind of interaction with him, perhaps sharing future knowledge about threats to hero society. His mother would also be someone he interacts with, as she is an important part of his life.
Perhaps the plot of this 'future Naruto goes to the past fanfic' is centered around Naruto's desire to reunite with his parents. In the future, he realizes that by going back in time, he could meet them when they were alive. So, he travels to the past. Once there, he not only gets to meet his parents but also tries to change some things in the past that would lead to a better future for the entire Hidden Leaf Village. He might use his knowledge of future threats to prepare the village earlier. He could also try to influence his younger self's relationships with other characters, like Sasuke. Maybe he tries to prevent Sasuke from leaving the village in the first place by showing him a vision of the future if he does leave. This would be a very emotional and action - packed plot, full of opportunities for character development and exciting twists.
Well, it could be that Izuku goes back in time and tries to change the way he became a hero. In the original story, he had a tough time at first. With Momo by his side this time, things could be different. Momo might be able to provide resources or support that Izuku didn't have before. And as Izuku tries to change his own hero journey, he might also end up changing the entire hero society in ways he didn't expect, all while building a deeper relationship with Momo.
The plot could be that Bell is sent to the past to observe Carsile for some unknown reason. As Bell follows Carsile around, they start to interact more. Carsile may initially be suspicious of Bell but then realizes that Bell has no ill intent. Their relationship could develop into a friendship or something more, all while they are navigating the complex world of vampires in the past, with different vampire clans and their own rules.
One possible plot could be that godlike Izuku uses his newfound powers to go back and prevent certain tragedies in the past. For example, he might stop the initial attack that led to so much chaos in his world. He could also use his knowledge of the future to help his friends earlier, like giving Midoriya tips on how to control his quirk better from the start.
Maybe Naruto discovers some dark secrets about his family that completely shatter his positive outlook. For example, if he finds out that his father was involved in some unethical experiments. This revelation could send him into an emo state where he questions his identity and purpose.
A great plot twist could be that Harry and Luna's marriage is part of a prophecy that no one knew about. As they are in hiding, they start to have strange dreams that lead them to uncover this prophecy. It turns out that their union is crucial for the balance of the wizarding world. But there are those who want to stop it, so they have to fight against not only the threat of discovery but also those who oppose their prophesied union.