There is no such appropriate fanfiction. In the Star Wars universe, which Anakin belongs to, there are certain values and moral codes. This kind of plot would completely shatter the integrity of the story and the characters, and it's not something that the fan community or the general public would tolerate.
It's a very bad and unethical idea. It might be the product of a person's deviant mind, but this has no place in the world of fanfiction. Fanfiction should be about building on the positive aspects of the original work, exploring character growth, relationships in a proper way, not creating such immoral and repulsive scenarios. In the case of Anakin, his story is about his fall to the dark side and redemption, not something as immoral as this.
A fanfiction could center around Shmi's relationship with the other slaves on Tatooine. She could be shown as a source of hope and strength for them. She might be involved in some sort of slave - led resistance movement, using her wits and kindness to rally others. This would give a different perspective on the often - overlooked Tatooine slave community in the Star Wars universe.
There could be a hermit or an outcast Jedi or Sith on Dathomir. Anakin meeting this character could lead to a great revelation. Maybe this character has been in hiding on Dathomir for years, observing the balance of the Force there, and Anakin's arrival disrupts that balance and forces an interaction that changes Anakin's perspective on many things.
He could interact with Luke Skywalker. Imagine Anakin finally getting to meet his son as an ally in the Rebellion. They could train together and share their knowledge of the Force.
Anakin could be angry. He might see his other self as a threat, especially if one represents his dark side and the other his light side. There could be a power struggle between the two versions of himself as they try to assert dominance over the other. Or he could be curious and eager to learn from his other self, about what he could have been or what he can still become.
Anakin could encounter future versions of his friends or enemies from the past. For instance, he could run into a future version of Obi - Wan Kenobi who has also somehow traveled to that future time. Or perhaps he meets a new generation of Sith who have different motives and methods compared to the Sith he knew in his own time. They could challenge his understanding of the dark side and the Force in general.
In some fanfics, it could be Anakin's own self - destructive tendencies. He has made so many bad choices in his life that in the end, his own actions lead to his death. For example, if he continues to pursue the dark side without fully understanding its consequences, it could ultimately consume him and lead to his demise.
Often, their interactions start from a place of conflict. Anakin's past actions as Vader create a barrier. However, as the story progresses, they may start to understand each other more. For example, Anakin could reveal his true self to Leia, the self that was once a good Jedi. This could lead to moments of deep connection and mutual respect between them. They might also have to work together against a common enemy, which would force them to communicate and cooperate, leading to a more developed relationship.