One key theme is often the exploration of the gray area between good and evil. When Obi - Wan joins Dooku, it challenges the traditional view of the Jedi as pure good and the Sith as pure evil.
The scenario of Obi Wan joining Dooku would be quite dramatic. It could potentially rewrite the entire fate of the galaxy. Maybe Obi Wan would have different motives or be under some kind of mind control.
If Obi-Wan joined Dooku, there might have been a power struggle between them. Also, it could have affected the relationships with other characters and the balance of the Force in unexpected ways.
One popular 'obi wan joined dooku fanfic' might be 'The Unlikely Alliance' where Obi - Wan's journey with Dooku is filled with political intrigue and moral dilemmas as they try to reshape the Jedi Order. It's a great read if you like exploring different character dynamics.
There may not be mainstream, widely - publicized fanfictions with Obi - Wan gagged. But some independent writers might have created stories around this concept. It could be that in a story, Obi - Wan is gagged during a diplomatic mission turned hostile. His captors want to silence him to gain an advantage in negotiations. This could lead to a complex plot where his friends have to use their wits and skills to free him and resolve the diplomatic crisis.
Count Dooku could be a prominent character. Given his own fall to the dark side, he might be a mentor or a rival to Obi - Wan in the fanfiction. Mace Windu is another one. His strict view of the Jedi Code and his relationship with Obi - Wan could be explored in the context of Obi - Wan's turn. And of course, there are the various Sith Lords and dark side adepts that might surround Obi - Wan, either as allies or enemies in his new dark side life.
Often, Obi - Wan uses his mastery of the Force. He might use a Force - push to overpower his guards when they least expect it and then make a run for it. He could also use a mind trick on a high - ranking captor to order his release.
Jon Snow could be one. He has a sense of honor and leadership, much like Obi - Wan. They could bond over their shared values and Jon could learn from Obi - Wan's wisdom. Another could be Daenerys Targaryen. She is on a quest for power and Obi - Wan might see her as someone who needs guidance to use her power responsibly.
He might have adventures related to understanding the unique culture of the all - female planet. For example, he could learn about their social structure, which may be centered around female leadership and cooperation. Maybe he would encounter some challenges in communication as their values and ways of thinking could be different from what he's used to in the Jedi Order.