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.
Well, perhaps Anakin could bring his unique combat skills to the Rebellion. He might use his knowledge of the Sith and the Empire's ways to help the Rebels in strategic planning.
In this fanfiction, Anakin could interact with the spirit of Yoda. Yoda has a connection to both Anakin and the events in 'The Last Jedi'. Their exchange could be really interesting as Yoda has his own wisdom to impart and Anakin has his own experiences. Also, Anakin might interact with Leia. He knew her well in his own time, and in 'The Last Jedi' she is still an important figure. He could be proud of her leadership in the Resistance or have some concerns about the path she's leading her people on.
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.
Obi - Wan Kenobi would be one. He's Anakin's master, and in the fanfiction, he might suspect something about their relationship and have to decide whether to support them or uphold the Jedi Code.
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.
In fanfiction, Anakin joining the Dark Side often has complex motives. It could be due to his fear of losing Padmé. The Dark Side might seem to offer him a way to save her, so he succumbs to its power.
In some fanfiction set in a more peaceful time, they might interact with a lot of tender moments. They could have picnics and long walks, just enjoying each other's company and having deep conversations about their future.
It could be a new and powerful dark side user. This character might have emerged from the shadows, having trained in secret, and their sole purpose is to take down Anakin as he is seen as a threat to their plans for galactic domination.
He might interact with it in a way that allows him to gain new Force abilities. For example, the Star Forge could amplify his connection to the Force, enabling him to perform feats he couldn't before.
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.