I don't think there would be a lot of fanfiction like that. Hinata has always been loyal to Naruto. In the Naruto universe, their relationship is based on a lot of development and mutual understanding. So it's hard to imagine a scenario where Hinata would cheat on Naruto with Boruto in a legitimate fanfiction.
Often, they are shown as more mature and experienced. Naruto is still the strong Hokage but also a loving father. Hinata has become more confident in her role as Naruto's wife and as a mother. For example, she might be more assertive in decision - making for the family.
Websites like FanFiction.net and Archive of Our Own (AO3) are great places to start. They have a large collection of fanfics related to Naruto and his family.
There are many fanfics that explore the relationship between Naruto and Hinata further. Some might focus on their daily family life with Boruto and Himawari, like how they deal with Boruto's rebellious phase while still maintaining a loving family environment.
Well, Boruto might be really embarrassed. He could quickly turn around and pretend he saw nothing. After all, walking in on his parents like that would be quite awkward for a kid.
One idea could be a story where Boruto and Himawari discover an old lemon tree in the Hidden Leaf Village. Naruto and Hinata then tell them about a legend that the tree was planted by the first Hokage. The kids decide to take care of the tree and throughout the process, they learn more about their family's history and the importance of nature in their village.
One interesting plotline could be Naruto and Hinata going on a secret mission together in Boruto's time. They get to show off their teamwork skills that have only gotten better over the years.
On the other hand, it could also bring the family closer. If they handle the situation well, by having an open conversation about it, Boruto could feel more included in his parents' relationship. He would understand that his parents have a special bond, and this could make him respect them more as a couple and as his parents. It could also lead to more family hugs and affectionate moments in general, as Boruto might be inspired by his parents' love.
Family could be the main theme. Naruto grew up without his parents, and Hinata has a strong family bond in the Hyuga clan. They could show how important family is in Boruto's life. Boruto might struggle with living in his father's shadow, but his family, especially Hinata's love and support, would help him find his own path.