One interesting storyline could be that Female Obi - Wan takes on a more maternal role towards Anakin and Ahsoka. She could be the one to help Anakin better control his emotions, which was a struggle in the original story. For Ahsoka, she could train her in a different way, perhaps focusing more on the spiritual side of the Force. This would change the dynamics of their relationships and lead to new adventures where they work together more harmoniously as a family unit.
Female Obi - Wan might develop a more nurturing side. Instead of the more stoic Obi - Wan we know, she could be more openly affectionate towards Anakin and Ahsoka. Anakin, with this different influence, might not be as quick to turn to the dark side. He would have more emotional support. Ahsoka would grow to be more confident, as she has two strong role models in Female Obi - Wan and Anakin. She would be able to learn from their different styles of using the Force.
One possible storyline could be that the female Obi - Wan discovers a lost child during a mission. Anakin and Ahsoka then help her take care of the child. They face many challenges like protecting the child from the Sith. As they do this, they also learn important lessons about family and loyalty.
The presence of a child would likely make their relationships more nurturing. Female Obi - Wan might take on a more motherly role towards the child, and Anakin and Ahsoka would be influenced by her example. They would become more protective of each other as they are all responsible for the child's well - being.
In many fanfictions, Obi - Wan might use his deep knowledge of the Force. He could reach out to Anakin through a Force - bond, calming his inner turmoil and guiding him away from the dark side. For example, he might remind Anakin of their past together, their training, and the values they once held dear.
Maybe the Force itself could be an antagonist in a sense. The light side of the Force would be trying to pull Obi - Wan back from the dark side. And the natural balance of the Force that doesn't want too much dark side influence could be working against Anakin's plan to turn Obi - Wan. So the Force, in its own way, could be the main antagonist.
One common theme is mentorship. Obi - Wan is often portrayed as a mentor figure to Ahsoka, much like he was to Anakin. For example, in many fanfics, he teaches her more advanced Jedi techniques or imparts wisdom about the Force.
In this fanfic, perhaps Ahsoka and Obi - Wan are drawn to a powerful Force - related artifact. As they search for it, they encounter various enemies who also want to possess it. They must use their combined Jedi abilities to fend off these foes. Obi - Wan's experience in handling such dangerous situations and Ahsoka's unique perspective as a former Padawan who left the Order would make for an exciting story.
In a fanfic, Ahsoka and Obi - Wan could be on a diplomatic mission gone wrong. They end up in the middle of a civil war on a planet. They have to work together to bring peace. Ahsoka might be more inclined to side with the rebels as she has seen the flaws in the Jedi Order, while Obi - Wan would be trying to find a more diplomatic solution that adheres to the Jedi Code. Their different perspectives would create an interesting dynamic throughout the story.