Their relationship might become stronger. Going through time travel together, facing new and old challenges, would force them to rely on each other more. They would see different sides of each other and gain a deeper understanding.
I think Yamato could be a good choice too. His ability to control wood style could be really useful in the past. Maybe he could time - travel to help with the growth of the first trees in the ninja world or something like that. And his calm and collected nature would help him adapt to the new situation in the past.
Another plot could be that they travel to a time during a great war in the past. In their ANBU capacity, they join the fight on the side of the Leaf. Along the way, they meet younger versions of their own teachers and comrades. They might discover secrets about their families or clans that were lost over time. This time travel experience could also make them re - evaluate their relationship with each other and the Leaf Village as a whole.
A really interesting fanfiction idea could be that Sasuke in the Anbu discovers a conspiracy within the village that even Naruto as Hokage is unaware of. Sasuke then has to decide whether to go to Naruto directly or investigate further on his own. This would create a lot of tension and drama, as Sasuke's loyalty to the village and his relationship with Naruto are both at stake.
One possible scenario in such a fanfiction could be Naruto as Hokage having to deal with political situations in the village while Sasuke, as an Anbu, is sent on dangerous covert missions. Naruto might rely on Sasuke's intelligence and combat skills from his Anbu work to protect the village from external threats. They could also have some internal conflicts due to their different roles and the secrecy associated with Sasuke's Anbu work.
Self - discovery is a significant theme. When Naruto travels back in time as an ANBU, he sees himself and the world around him in a new light. He discovers new aspects of his abilities and his personality. Additionally, the theme of secrecy and trust is common. As an ANBU, he has to keep his identity and mission a secret, but he also has to figure out who he can trust in this new timeline. This adds a layer of mystery and tension to the fanfictions.
A common theme is redemption. Naruto often goes back in time to correct his past mistakes or the mistakes of others. For example, he might try to save a character who died in the original timeline. Another theme is power - growth. Since he has knowledge from the future, he can train more effectively in the past and become even stronger.
I really liked 'Anbu Naruto's Time - Twisted Adventure'. In this fanfic, Naruto goes back in time and joins the Anbu much earlier. It's full of action and there are some really cool fight scenes. The author also does a great job of exploring how Naruto's time travel affects the relationships he has with other characters.
One possible scenario in such a fanfiction could be that the Anbu Commander travels back in time to prevent a major tragedy in the Naruto world. Maybe he goes back to stop the Uchiha massacre. He could use his knowledge of future events and his Anbu skills to change the course of history.
Another theme is the exploration of the ANBU world itself. These fanfictions often describe the dark and dangerous nature of the ANBU, the strict training, and the complex hierarchy. Sasuke's experience in this world is then contrasted with his relationship with Naruto, who represents a more positive and hopeful side of the Naruto universe.