Some fanfictions might explore a situation where Anakin is being manipulated by a powerful dark force or a Sith Lord. This dark influence makes him see Obi - Wan as a threat to a grander plan. So, he kills Obi - Wan as part of this dark scheme, not fully in control of his own actions. And this can lead to a lot of complex emotions and plot developments later in the story, like Anakin's further descent into darkness and how the other characters react to Obi - Wan's death.
The most obvious implication is the destruction of Obi - Wan's world. Obi - Wan would be devastated and likely turn all his efforts towards revenge. This would change his character from a calm and collected Jedi to a more vengeful figure.
The impact would be far - reaching. For the Jedi, Obi - Wan's death at Anakin's hands would be a major blow to their morale. His teachings and wisdom were highly valued. Without him, the younger Jedi might lose their way. In the greater Star Wars universe, it could change the course of the war. Anakin's actions could lead to different alliances being formed. Some planets might side with the Sith out of fear or a belief that they are the new power. It would also affect the relationship between the Force - sensitive characters. Those who were close to Obi - Wan might seek revenge, while others might try to redeem Anakin.
Padmé would be devastated. She loved both Anakin and Obi - Wan. Her relationship with Anakin would become extremely strained, if not completely broken. She might try to stop Anakin from further falling into the dark side or, in her grief, turn against him.
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.
It might also cause a huge shift in the balance of the Force. Anakin's act could be seen as a major step towards the dark side, and it could start to tip the scales in favor of the Sith. The galaxy would then be plunged into even more chaos as the Sith gain more power and the Jedi are left reeling from the shock of such a heinous act.
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.