Well, first of all, Ahsoka is known for her loyalty to the Jedi values. So even when captured, she can uphold those values in her thoughts and actions. She might try to convert or at least influence her captors in a positive way if possible. Also, her combat skills and quick thinking can be shown in a different context. For instance, if she's in a fight - like situation within the captivity area, she can use her skills to defend herself and others. Moreover, her sense of humor can still be there, which is a part of her character, like making witty remarks to her captors to show that she's not intimidated.
One plot idea could be that Ahsoka is captured by a group of Sith cultists. They plan to turn her to the dark side by using ancient Sith mind - control techniques. Another could be she's captured by a powerful crime syndicate who wants to ransom her to the Jedi Council. Maybe she's captured during a mission on an unknown planet and has to use her wits to escape while also uncovering a greater conspiracy.
Ahsoka can be developed by showing her growing understanding of Anakin and Padme's relationship. For example, she could start off being a bit naive about love and then gradually realize the depth of their connection. Also, her skills in the Force could be enhanced through her experiences with them. Maybe she has to face a powerful enemy alone while protecting Anakin and Padme, and this forces her to dig deeper into her Force abilities.
Her opponents could be the Separatist forces. Umbara was a planet involved in the Clone Wars, so it's likely that battle droids and their commanders would be her foes. They would be trying to gain control of Umbara's resources or strategic locations.
In some versions, it could be a former ally who has been corrupted. This character, once on Ahsoka's side, has now turned against her due to some external influence or personal greed. Their knowledge of Ahsoka's strengths and weaknesses would make them a formidable opponent, and they could be the one to cause her death.
Well, it could be that she was lured into a false sense of security and then ambushed. Or perhaps she made a mistake that gave her captors the opportunity to seize her.
She could be developed through her interactions with Sith apprentices. Maybe she tries to redeem one of them, showing her compassion and strong will. This would also show her growth from her Jedi training.
Ahsoka could become more cautious and distrustful. For example, she might be hesitant to engage in combat or trust other Jedi as easily after being hurt by Anakin. This change in her attitude would be a significant part of her character development.
To write a believable character arc for Ahsoka, start with her existing traits. She's strong - willed and independent. Maybe have her start the fanfic being a bit overconfident and then face a series of setbacks that make her question herself. For example, she could lose a battle against a new enemy and then have to relearn her fighting techniques.
I'm not sure specifically what this fanfic is about as it's not a mainstream or widely known official story. It could potentially be a creative and perhaps unorthodox exploration of the character Ahsoka in a situation related to Jabba in a fan - created narrative.
Ahsoka might develop by learning from the Sith's combat techniques without succumbing to the dark side. This could give her an edge in future battles and also show her adaptability. She could study their fighting styles and incorporate elements into her own, all while maintaining her moral compass. In a more complex way, she could start to understand the Sith's motives, not to justify them, but to better predict their actions. This new understanding could shape her into a more strategic and wiser character in the fanfiction.