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.
In the fanfiction, it might be that Anakin has been poisoned. As the poison takes effect, he collapses into Obi - Wan's arms. Or perhaps Anakin has just learned some devastating truth about his past or the Jedi Order that shocks him to the point of physical collapse.
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.
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.
One interesting Anakin and Obi - Wan fanfiction could be about their early days as master and apprentice. It might explore how Obi - Wan tried to teach Anakin the ways of the Force, like in a story where they go on a mission together to a strange planet. Anakin might make some rookie mistakes and Obi - Wan has to patiently guide him. Another could be a story set during the Clone Wars, where they face a difficult battle and have to rely on each other's skills to survive. There are also fanfictions that explore what could have happened if Anakin had not turned to the Dark Side, and how their relationship would have continued to grow and develop in a positive way.
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.
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.
In many Obi - Wan and Anakin fanfictions, the theme of mentorship is key. Obi - Wan's role as Anakin's teacher and guide is central. Also, the idea of destiny plays a part. Whether they can fulfill their destinies together or separately is a common plot point. Additionally, the theme of sacrifice is present, as both characters are in a position where they may have to sacrifice for the greater good or for each other.
The struggle between the light and dark side is also common. Since we know Anakin's fate, fanfictions often play with the idea of Obi - Wan trying to keep Anakin on the path of the light. It's about the internal and external battles they face in relation to the Force.
A possible plot twist could be that Obi - Wan was actually faking being turned to the dark side all along. He was trying to infiltrate the Sith ranks to learn their secrets and stop them from within. He lets Anakin think he has succeeded in turning him, but in reality, he is still a Jedi at heart and has a plan to take down the Sith.
One possible consequence is that the Jedi Order becomes stronger. With Anakin remaining on the light side, his power is added to the Jedi's arsenal. They can better face the threats from the Sith and other dark forces.