Definitely his professors when they were younger. For example, Snape as a student. This would be a strange interaction considering their relationship in Harry's own time. He could also meet Hagrid when he was just starting out at Hogwarts. And there's a good chance he'd run into Sirius and Remus during their school days, and perhaps form new friendships or alliances with them in the past.
One possibility is that Harry would use this opportunity to change certain events in the past that had a negative impact on the baby or its family. He could also form a special bond with the baby. Maybe he'd introduce the baby to some of his friends from his own time who are also in the past now. This could create a whole new set of relationships and storylines in the fanfiction.
Well, he might try to expose Voldemort earlier in his rise to power. Harry could use his knowledge of Voldemort's Horcruxes to start looking for them right away. Also, in the past, he could form new alliances. Maybe he teams up with some of the older generations of wizards and witches who were previously unaware of Voldemort's true intentions. And his presence in the past could cause a stir in the wizarding world as people try to figure out who this mysterious boy is with such strange knowledge.
One great 'Harry ends up in past with Teddy' fanfiction is 'Time's Twist'. In it, Harry's journey in the past with Teddy leads to some interesting changes in the wizarding world's history. They form a strong bond and together face many challenges that were previously unknown in the original timeline.
Well, the storyline could change in many ways. For example, if Naruto goes back in time, he might prevent some of the tragedies that happened in the original story. He could stop the Uchiha massacre, which would completely change Sasuke's story arc. Also, he might be able to train with the earlier versions of his teachers and learn new jutsu much earlier, making him even more powerful in the future timeline.
In most of these fanfictions, once Harry and Teddy are in the past, they first have to figure out how to blend in. Teddy, being younger, might rely on Harry a lot. Harry, with his knowledge from the future, will start to make decisions that could change the course of history. For example, he might introduce new magical concepts earlier or prevent certain dark wizards from rising to power. As the story progresses, their relationship deepens as they face challenges together, like dealing with the suspicions of other wizards in the past who notice their strange behavior or knowledge. And there are often emotional moments where Harry reflects on his own past and how this new experience is changing his perspective on things.
One great fanfiction could be 'Time's Twist in Slytherin'. In this story, Harry's time travel leads him to be sorted into Slytherin. He has to navigate the complex politics and relationships within the house. It shows a different side of the Slytherin characters and how Harry's presence changes things. He forms unexpected alliances and discovers secrets that were long buried.
In a fanfiction where Harry Potter is on Asgard, he would face many challenges. His magic might not work in the same way as it does on Earth. He'd need to adapt quickly. He could also be seen as a threat by some Asgardians. However, his courage and quick thinking, which he developed in his battles against Voldemort, would help him survive. He might even teach the Asgardians some things about the power of love and friendship, which are important in the wizarding world.
Another possibility is that Harry ends up married to a Muggle he meets in Vegas. This would be really interesting as he has to navigate the differences between the wizarding world and the Muggle world while being married to someone who has no idea about magic at first. It would be a story of discovery and adjustment for both Harry and his new spouse.
In the past, Naruto could gain the ability to change the fates of characters. This isn't exactly a traditional ability, but it's very important. By changing the past, he could make his friends stronger in different ways. For example, he could ensure that Rock Lee gets proper medical treatment earlier, which might allow Lee to develop even more powerful taijutsu techniques. This, in turn, would change the dynamics of battles and how Konoha as a whole approaches combat.