In such a fanfiction, Naruto could share his old ninja tales with Boruto. Maybe he'd tell about his battles against the Akatsuki. Boruto would be in awe, seeing his father in a new light.
Definitely Naruto and Sasuke. In a 'past Naruto meets Boruto' fanfiction, if Sasuke was also involved in the meeting, it would be really cool. Boruto could see the dynamic between his father and Sasuke, two of the most powerful ninjas. He could learn about their past rivalries and how they overcame them to become allies. It would be a great opportunity for Boruto to understand the importance of having a strong bond with your comrades.
His presence could create a butterfly effect. Small actions he takes might change the future. For example, if he teaches a young ninja a new technique, it could change how battles are fought in the future.
At first, they might be a bit suspicious of each other. Boruto has heard stories about Kurama but has never really interacted with him. Kurama, on the other hand, might be cautious as Boruto is Naruto's son and not sure if he can be trusted. But as they spend more time together, they could start to respect and rely on each other.
Well, Boruto could be very inspired. Naruto's past is full of struggles, from not having parents around to being constantly underestimated. When Boruto discovers this in the fanfiction, he might realize that his own problems are not as big as he thought. He could be motivated to work harder and not take for granted the things he has, seeing how much Naruto had to fight for everything.
Perhaps in this fanfiction, Neji is brought back to life somehow. Boruto, being curious, approaches Neji. Neji then discovers how much the ninja world has changed since his time. Boruto could show Neji the new technology in the Hidden Leaf Village. Neji might be a bit shocked at first but then start to see the potential in these new things. They could also form an unlikely friendship, with Neji guiding Boruto in matters of honor and discipline, which are values Neji held dear.
Maybe in the fanfiction, Jiraiya is summoned by a new jutsu gone wrong. When he meets Boruto, he mistakes him for Naruto at first. As they interact, Jiraiya realizes Boruto's own unique struggles. He shares his wisdom about handling the expectations that come with being a ninja from a famous family, and Boruto learns a great deal from him.
One possibility is that Kurama senses a new threat approaching and decides to team up with Boruto. They could go on an adventure together, with Boruto having to learn how to work with the powerful and sometimes grumpy Kurama. Along the way, Boruto could learn more about the nature of chakra from Kurama, which would be a great development for his ninja skills.
In a 'Boruto meets Kurama' fanfic, Boruto might learn about Naruto's past with Kurama. Maybe Kurama would share some of his old battles with Boruto, giving Boruto a new perspective on his father's journey.
Another plotline might be that Kurama, having been with Naruto for so long, has some unfinished business related to the ninja world. He enlists Boruto's help to complete it. This could involve uncovering secrets from the past or righting some old wrongs. And along the way, Boruto would grow as a ninja and as a person.