A really engaging plotline could be about Naruto's discovery of his Uzumaki heritage. In the fanfiction, it could go deeper into how he found out about his family's past, the significance of the Uzumaki clan, and how it influenced his growth as a ninja. This could include his exploration of Uzumaki jutsu and the secrets his family held that he was unaware of before.
Another plotline might focus on Naruto's relationships with his classmates in the academy that we didn't see much of in the original series. For example, how he interacted with Shikamaru or Choji before they became good friends. It could show some untold stories of their early rivalries or alliances.
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 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.
Well, a cool plotline could be Boruto having to deal with the combined enemies of his father and Sasuke. Enemies from Naruto's past might target Boruto to get revenge on Naruto, and at the same time, there could be remnants of Sasuke's old foes as well. Also, in the fanfic, it would be interesting to see how Boruto's training is different. Maybe he has both Naruto and Sasuke as his mentors, and they constantly bicker about the best training methods, which adds a bit of humor to the story.
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.
One interesting plotline could be Naruto using his future knowledge to prevent certain tragedies in the past. For example, he might stop the Uchiha massacre. Another could be him training his younger self more effectively, leading to a much stronger Naruto earlier on.
In some 'Naruto appears in the past' fanfictions, Naruto might go back to train with the early Hokages. He could learn new jutsus directly from Hashirama and Tobirama, which would make him even more powerful. This would also change his relationships with his friends in the present as he would come back with enhanced skills and knowledge.
Another aspect could be Naruto's training with Jiraiya. Boruto might read in the fanfiction about how Naruto traveled with Jiraiya, trained hard, and grew not just in strength but also in character. Jiraiya taught Naruto many important lessons about life and being a shinobi. This could give Boruto a newfound respect for his father's journey to becoming the Hokage.
Another plotline could be Naruto influencing the relationships between characters. He could try to bring Sasuke back to the village much earlier in the timeline, preventing a lot of the pain and conflict that occurred between them. Or he could help his parents, Minato and Kushina, in ways that would change the future of the entire Hidden Leaf village.
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.
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.
One possible plotline could be a powerful new enemy emerging that neither Naruto nor Boruto can defeat alone. Maybe an ancient evil that has been awakened and has powers far beyond what the ninja world has ever faced. Their deaths could be a sacrifice to seal this evil away, protecting the village and the world at large.