She could be a hero. Maybe she discovers a new way to use the Force that could save the galaxy from the Sith. For example, she might develop a technique to heal the dark side - tainted planets.
If that were the case, the galaxy's fate would likely have been very different. Maybe they could have avoided the conflicts and tragedies that came with Anakin's Jedi path. But who knows how other forces would have shaped events without the Jedi's influence.
In a more political - intrigue - based fanfiction, Anakin's little sister could be involved in the Senate. She could be a voice of reason among the corrupt politicians. As she uncovers a conspiracy within the Senate that is linked to the Sith's plan to take over the galaxy, she has to team up with some unlikely allies, including some Jedi, to stop it. And all the while, she has to deal with the fact that her brother Anakin is being manipulated by the Sith.
One interesting Anakin Skywalker's sister fanfic could be 'The Lost Skywalker'. It delves into the idea of his sister having been hidden away all this time. The story shows her emerging and how she copes with the new world she discovers, as well as her relationship with Anakin.
One great fanfiction could be where Anakin's little sister has hidden Force abilities. In this story, she might secretly train with Obi - Wan Kenobi, away from Anakin's knowledge. This could add an interesting dynamic as she tries to balance her loyalty to her brother and her new - found Jedi path.
Anakin not going to the dark side could have prevented many conflicts. Maybe he would have found a way to peacefully resolve issues instead of resorting to violence. It would have changed the course of the entire Star Wars universe for the better.
Had Anakin killed Palpatine, it's likely that there would have been chaos initially. Different factions might have vied for control. But in the long run, it could have paved the way for a new era of hope and stability in the Star Wars galaxy.
Anakin would be completely shattered. Ahsoka was one of the few people he truly cared about deeply. Her death in his arms would make him question his abilities as a Jedi and a protector. He might feel a deep sense of rage towards whatever or whoever caused her death, which could start to cloud his judgment and lead him down a darker path.
There could be a hermit or an outcast Jedi or Sith on Dathomir. Anakin meeting this character could lead to a great revelation. Maybe this character has been in hiding on Dathomir for years, observing the balance of the Force there, and Anakin's arrival disrupts that balance and forces an interaction that changes Anakin's perspective on many things.