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.
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.
Well, one possibility could be that Anakin goes to Dathomir in search of some unique Force knowledge. Maybe he's heard rumors about ancient Sith secrets there.
Obviously, he would interact with his younger self. He could try to guide his younger self, offer advice on how to control the Force better and how to deal with the temptations of the Dark Side.
In a fanfic, Sith Lords are likely candidates to kidnap Anakin. They might see him as a valuable asset or a threat to their plans and decide to take him captive. For example, Darth Maul could try to kidnap Anakin to get revenge on the Jedi or to use him in some dark scheme.
Obviously, the members of the Jedi council would be main characters. Yoda, for instance, would play a crucial role. His wisdom and experience would lead him to question Anakin's motives and the truthfulness of what he says.
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.
Anakin's mother, Shmi, would likely be a very important character. Since Anakin is not being trained as a Jedi, he would be more focused on his family. Shmi's influence on Anakin would be even greater, and she could play a role in guiding his actions and decisions, especially in relation to the local community on Tatooine.
Well, in the fanfic, Anakin's skills would be a huge asset or a threat to the First Order. If he decides to join them, his combat abilities and knowledge of the Force could make the First Order even more powerful. However, if he realizes that they are on a wrong path, like the Sith - influenced path he once walked, he might use his skills to undermine the First Order from within. He could start by influencing the stormtroopers, showing them that there are other ways than blind obedience.
One possible plot could be Anakin going back to a time before his fall to the dark side. He could use his knowledge of the future to prevent certain tragedies. For example, he might stop the Clone Wars from starting, thus saving countless lives. This would also give him a chance to redeem himself and perhaps even change the fate of the Jedi Order.
Well, he could either try to take over the First Order with his powerful Force abilities. Maybe he would see it as a new tool to gain more power and control in the galaxy.