Sansa would be more assertive in her political moves. She has seen the consequences of inaction and bad decisions in the future. So, she would be quicker to make decisions that benefit her family and herself. For example, she might be more involved in military strategies to protect the North.
One idea could be that Sansa goes back in time and tries to prevent the Red Wedding. She could use her knowledge of the future to warn her family and change the course of that tragic event.
One of the most common characters is definitely her father, Ned Stark. Since Sansa has the chance to go back in time, she would likely interact with him a great deal to try and change the course of events. Another is Arya. They could team up and use their combined knowledge and skills to make a difference. And of course, Littlefinger. Sansa would be more aware of his manipulations this time around and might interact with him in a more strategic way to outwit him.
In this type of fanfiction, a great plotline could be Sansa influencing the political landscape much earlier. She could use her knowledge of how certain houses would act in the future to her advantage. For example, she could form early alliances with houses that she knows will be important later. She might also try to change the way her family views certain people. Maybe she could convince her father to see Littlefinger's true nature much sooner. This would not only protect her family but also change the entire power dynamic in Westeros.
She might change the course of events in the past. For example, she could prevent some tragedies that happened to her family. Maybe she would use her advanced knowledge to help her father, Tony Stark, earlier in his life, which could potentially lead to a different future for the Avengers and the whole world.
One possibility is that Tony could prevent some of the major disasters that occurred in the Marvel Universe. For example, he might stop Loki from getting the Tesseract in the first Avengers movie. This would change the entire timeline and prevent the destruction of New York City that followed. Another thing is that he could meet his parents at a different time, which would be an extremely emotional encounter for him. He might also be able to use his advanced knowledge to help Howard Stark with some of his early inventions, potentially accelerating the technological progress of that era.
Well, a cool plot in an Arya Stark goes back in time fanfiction could be that she travels back to the time when the White Walkers first started to become a threat. Arya, with her knowledge of the future, could try to find a way to stop them from ever becoming such a powerful force. Maybe she'd team up with some of the early Night's Watch members in a way that changes the entire history of the war against the dead. Also, she could go back to the time when the Starks were still in a more powerful position in the Seven Kingdoms and use her skills to strengthen their alliances.
Time travel can make Sansa more confident. When she travels back, she has the knowledge of future events. So she can be more assertive. For example, she won't be as easily manipulated as she was before. She becomes a stronger character as she tries to change the fate of her family.
In 'dark Sansa Stark fanfiction', Sansa can be more assertive. Unlike in the original where she is often a victim of circumstance, here she takes charge. For instance, she might not wait for others to rescue her but actively plot her own escape and revenge.
Another theme is power. She may use her knowledge of the future to gain power or influence in the past. This could be political power, like influencing the decisions of the lords, or personal power, such as learning combat skills earlier. It's about how she can rewrite her own story with the advantage of hindsight.