One unique idea is that Boruto travels back in time to the era when Naruto and Hinata were young. He gets to see their struggles first - hand and understand them better. It also gives him a chance to learn new skills and techniques from the past versions of his parents. Another idea is that Hinata has a secret mission that she has to keep from Naruto and Boruto, which creates a lot of suspense and drama in the fanfiction.
One idea could be to focus on their anticipation and preparation for the baby. They might discuss names, decorate the nursery, and Hinata could be shown getting advice from her family. Naruto, on the other hand, would be excited but also a bit nervous about becoming a father.
One idea could be a story where Boruto and Himawari discover an old diary of Naruto's from his early days as a ninja. Hinata helps them understand the context and the values their father held. They then use those lessons to face a new threat in the village.
In certain fanfictions, Hinata and Hanabi team up to go on a secret mission outside the village. This is different from the usual storylines in the Naruto canon and allows for new adventures and character development. They might face challenges that test their sisterly bond and ninja skills.
How about a story set in an alternate universe where Sasuke and Hinata are arranged to be married? Naruto then tries to break the arrangement because he realizes his true feelings for Hinata. This would create a whole new set of relationships and conflicts among the three.
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.
A possible plot is that Boruto discovers a hidden power within himself that was passed down from Naruto and Hinata combined. This power could put him in a dangerous situation as various factions might want to either control or eliminate him. Or perhaps Boruto has to deal with the pressure of being the son of the Hokage and a powerful Hyuga, and he decides to go on a journey to find his own identity away from his parents' shadows.
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 plot idea could be Boruto getting lost in a mysterious forest during a training exercise. Naruto and Hinata then team up to find him. Along the way, they face some old enemies or new threats that test their skills and their relationship as a couple. Another idea is that Boruto discovers a hidden power within himself that Naruto doesn't understand, and Hinata has to be the one to bridge the gap between father and son by helping Naruto understand Boruto's new ability.
A fanfiction idea could be that Hinata and Kiba are transported to a modern - day city. It would be interesting to see how they adapt to the new environment, with Hinata being shy around all the new things and Kiba being more adventurous, maybe getting into some comical situations as he tries to figure out modern technology.
A cool Naruto and Boruto fanfiction concept could be exploring the relationship between Sasuke and Boruto in more depth. Maybe Sasuke takes Boruto on a special training mission, similar to how he trained with Naruto in the past. This would allow Boruto to understand Sasuke's way of thinking and his past better. Also, a story where all the Konoha 11 have kids and they all train together could be interesting.