One possibility is that Aang goes back and tries to train the Avatar before him more effectively. This could lead to a stronger chain of Avatars and a more balanced world. He could also use his knowledge of the future to find new airbenders and save his own people from extinction much earlier.
Well, he could prevent some of the great disasters that happened in his world. Maybe he could stop the Fire Nation's initial attacks and change the whole course of the war. He might also be able to save some important characters who died in the original timeline.
Well, he could try to change all the tragic events that occurred in the original timeline. Maybe he'd prevent certain deaths or make different alliances. For example, he might approach Suzaku earlier and try to convince him of his true motives, thus avoiding their later conflicts.
Well, in this fanfiction, Aang might bring his airbending skills to Hogwarts. He could use his bending to create interesting effects during Quidditch matches, like using air currents to speed up the broomsticks. He may also interact with the Hogwarts students in unique ways, perhaps teaching them a bit about his culture from the Avatar world.
His relationship with Katara would be very different. He might be able to approach her in a more confident way, knowing what they've been through in the future. He could also avoid some of the misunderstandings that happened between them in the original timeline.
In this fanfiction, they could go on an adventure together. Jinora is a very spiritual person, and Aang was also deeply connected to the spiritual world. They could explore some ancient spiritual sites. For example, they might visit the ruins of an old air temple that Aang used to know but Jinora has only heard about in stories. There, Aang could share his memories of the place, and Jinora could gain a deeper understanding of airbender history.
He could change the course of major events. For example, he might prevent the rise of a powerful villain. This would have a huge impact on the entire galaxy. Maybe he could save some characters who died in the original timeline.
In this fanfiction, Sora might change key events. Maybe he could prevent some of the tragedies that occurred in the original story. For example, he could save a character who was lost before.
She might change important events in the wizarding world. For example, she could prevent certain dark magic incidents or save characters who died in the original timeline.
Well, he might meet his ancestors. It could be really interesting to see how they interact and what kind of influence he has on their lives. Maybe he'll change some small events that end up having a big impact on his present time.
In this kind of fanfiction, Jotaro might change some crucial events. For example, he could prevent some of his friends from getting hurt or even stop the main villain from starting their evil plans at an earlier stage. This would completely change the original story's timeline and create a whole new set of adventures and challenges for Jotaro and the other characters.
Well, he might prevent some of the villains from ever becoming powerful. For example, he could stop Frieza from getting all those power - ups.