Obviously, Naruto himself is a crucial character. His experiences and growth form the backbone of the story.
Sasuke is also likely to be an important character. His relationship with Naruto, whether it's a rivalry or a budding friendship in the past, can add a lot of depth to the fanfiction. And his own journey as a ninja from the Uchiha clan can be intertwined with Naruto's story in interesting ways.
Boruto might be in awe. He has always known his father as the Hokage, a powerful and respected figure. Seeing Naruto's past full of struggles, loneliness, and determination would make him realize how far his father has come. It could also make Boruto more empathetic towards Naruto and perhaps inspire him to be more like his father in some ways.
The Akatsuki members might also play a role. Since Naruto's past could be related to the larger shinobi world, the Akatsuki, with their various motives and powers, could be involved. For example, if Naruto's family had something to do with a powerful tailed beast that the Akatsuki is after, it would draw them into the story and make them important characters in this fanfiction.
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.
Sasuke is very likely to be involved. He has a close relationship with Naruto and is also an important figure in Boruto's life. His skills and knowledge could be crucial in any adventure or situation that Naruto and Boruto find themselves in. Another character could be Hinata. As Naruto's wife, she would surely be concerned about any strange events involving Naruto. And for Boruto, she is his mother, so she would play a role in any family - related or village - threatening situations.
One interesting plotline could be exploring Naruto's early days of training with Jiraiya in more detail. Maybe showing his struggles and how he overcame them during that time.
One of the most shocking revelations could be Naruto's early days as an outcast. Boruto, who has always known his father as a respected figure, would be surprised to learn that Naruto was once shunned by the village. Another could be the danger Naruto faced as a young ninja, like fighting against the Akatsuki. It's so different from Boruto's relatively more sheltered life in the beginning.
The most popular characters in 'Boruto meets Naruto fanfiction' are Naruto and Boruto, as they are the main focus of the concept. However, characters like Jiraiya can also be very popular. His connection to Naruto and his own legendary status add depth to the fanfiction. Shikamaru is another character that fans like to see in these stories. His intelligence and laid - back attitude can create some interesting interactions with the main characters.
Definitely Sasuke. His relationship with Naruto is so important in the original story, and when it comes to a fanfiction with kid Naruto and Boruto, Sasuke can act as a mentor or a challenging rival for both of them. Another great character is Hinata. She has a special connection to Naruto, and in a fanfiction, she could play a role in guiding Boruto or providing support to kid Naruto in a new situation.
Probably his time at the Ninja Academy. Naruto was always getting into trouble there, but it was also where he made some of his first friends. Also, his encounters with the Third Hokage could be shown. The Third Hokage was like a grandfather figure to Naruto, and their interactions were important in Naruto's development.
Boruto could have visions or dreams that show him Naruto's past. Maybe he enters a forbidden realm or unlocks a secret power that allows him to experience his dad's history.