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.
Perhaps he goes back in time and decides to train himself better. He could focus on mastering his emotions from the start, which might change the entire course of the Star Wars saga. He might also try to save his mother in a more effective way, without succumbing to anger and hatred.
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.
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.
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.
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.
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.
One possible plot twist could be Anakin going back in time and preventing his own fall to the dark side. He might use his knowledge of the future to avoid the events that led him to become Darth Vader. For example, he could avoid the temptation of Palpatine's offers. Another twist could be that his time - traveling disrupts the Force in unexpected ways, causing new alliances and enmities to form. Maybe he accidentally changes the course of other Jedi's lives, leading to a completely different galactic power structure.
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.
Anakin's time travel could lead to a completely different relationship with Padmé. He may realize that his obsession with her in the original timeline was part of what led him down the dark path. So, instead of repeating the same pattern, he could focus on being her friend and protector in a more platonic way. This new dynamic might not only change their relationship but also have a significant impact on Anakin's overall character development and his place in the galaxy.