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.
One popular 'naruto y hinata fanfic' is 'The Road to Happiness'. It beautifully portrays their relationship development, from Hinata's shy admiration to Naruto finally realizing his true feelings for her. Another great one is 'A New Beginning' which shows their life after getting married and facing new challenges together in Konoha.
In a good 'naruto y hinata fanfic', the characters need to be in - character. Naruto should still be the determined and sometimes clueless hero, and Hinata should be the shy yet strong - willed kunoichi. The story should also fit well within the Naruto universe, like having the right kind of jutsu and village settings. It should also have an engaging plot, whether it's about them going on an adventure together or just dealing with life in Konoha. For instance, a story where they team up to solve a mystery in the village can be really exciting.
Well, Naruto would be in a really tough spot. He loves his family and the village, and this situation would be a huge blow to his traditional values. He might struggle to understand how this happened between his son and Hinata. Hinata's family, on the other hand, would be protective of her. Her father would want to ensure that Boruto is taking responsibility. Boruto's friends would also be surprised. Some might be supportive, like Shikadai, who would try to see the good in the situation, while others might distance themselves at first, not sure how to handle this new and rather strange development.
One common theme is family. Since Hinata is Boruto's mother, many fanfics explore their family relationship, like how Hinata passes on the Uzumaki - Hyuga values to Boruto.
One possible plot could be that Boruto and Hinata have a deep emotional bond that develops over time. They face many challenges in the Hidden Leaf Village together, which brings them closer. As a result of their love and closeness, Hinata becomes pregnant, and then they have to deal with the reactions of their family and friends, like Naruto's reaction as Boruto's father and also how the village will perceive this new development.
One good theme could be their training sessions. Maybe Hinata is teaching Boruto some of her unique ninja techniques, like her gentle fist style. This can show their bond as teacher - student within the family.
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.
There are indeed some. Look for fanfics that are tagged as 'General' or 'All Ages'. These are likely to be free of any inappropriate content and can give you interesting takes on the characters of Boruto and Hinata, like their adventures together or how Hinata mentors Boruto in some aspects.