One possible plot twist could be that Jon Snow goes back in time and accidentally changes something that leads to a completely different future for the Stark family. For example, he might prevent Ned Stark from going to King's Landing, which would change the power dynamics in Westeros entirely.
His relationship with his father, Ned Stark, would be different. He could finally have a real father - son relationship instead of the hidden one. He might be able to learn from Ned and become an even better leader in the North.
Regarding his relationships with his friends and comrades, like Samwell Tarly, things would be altered. He might not meet Sam in the same way as before. And his relationship with Daenerys Targaryen could be completely different. Instead of meeting her during the events of the original story, he could meet her when she was much younger, and this could lead to a very different kind of relationship, maybe one that's more of a mentorship or a completely different romantic connection.
She could meet her younger self. This would be an interesting encounter as she could offer advice and guidance to her younger version. Maybe she could help her younger self to be less afraid of her powers.
The wildlings might be his allies. After all, Jon Snow has had interactions with them before. They are fierce fighters and could fight alongside him against a common enemy.
One possible plot could be Serena going back in time to prevent a major disaster in her world. For example, she might go back to stop an evil force from being released. In the past, she could meet younger versions of her friends and family, and they might not recognize her at first. She would have to use her knowledge from the future to try and change things for the better.
In the 'Petunia Goes Back in Time' fanfiction, she might try to change things that happened in the past. Maybe she wants to fix a mistake or prevent a bad event from occurring.
One possibility is that Draco could try to change his family's fate. In the original story, his family was on the wrong side of the wizarding war. By going back in time, he might attempt to steer them towards a more positive path, perhaps by preventing them from getting involved with Voldemort in the first place.
One common theme is redemption. For example, Rey might go back in time to redeem a character who has fallen to the dark side. Another theme is the exploration of 'what if' scenarios. Like what if Rey changed a certain event in the past, how would it affect the future of the Star Wars universe.
He might change the course of some major battles. For example, he could prevent the destruction of certain planets that were destroyed in the original timeline.