Well, in a fanfic where adult Naruto meets kid Naruto, it could be really interesting. Adult Naruto might be super proud of his younger self. He could give kid Naruto some tips on how to train better and how to deal with the loneliness he sometimes feels. Maybe he'd also tell him some spoilers about future events in a fun, teasing way.
Maybe they'd embark on a mission to stop a new threat that's related to Naruto's past. Kid Naruto would be all excited and ready to fight, while adult Naruto would use his wisdom and more powerful skills to lead the way. They could also meet some of Naruto's old friends as kids and have some really fun interactions.
Well, in such a fanfiction, Naruto might learn some new and cool ninja techniques from the toads. Maybe he'll have an adventure in the toad village and make new friends among the toads.
One possible relationship could be that of a student and teacher. Naruto could learn from the toads like he does from his human sensei. Another relationship might be friendship. They could become friends and support each other in different situations.
In a fanfiction where Boruto meets Kid Naruto, there could be a lot of chaos at first. Boruto might be shocked to see a younger version of his father. Kid Naruto would probably be curious about Boruto's strange clothes and gadgets. They could end up getting into some kind of adventure together, like facing a common enemy. Maybe Boruto would show Kid Naruto some of his ninja techniques from the future, and Kid Naruto would teach Boruto some of his own unique skills that he developed during his own time as a young ninja.
One interesting plot could be two Narutos from different timelines meeting. They could exchange stories about their battles and growth, and then team up to face a new, powerful enemy that neither could defeat alone.
Well, obviously Naruto himself would be the main character. There could also be his friends like Sasuke and Sakura, who might be involved in the interactions between the two Narutos. Maybe they are confused at first by seeing two Narutos, but then they help them figure out what's going on.
It could be the First Hokage, Hashirama. Naruto would be amazed by his powerful Wood - Style jutsu and his ability to create the village. He could learn about the original ideals behind the Hidden Leaf from Hashirama.
One possible plot could be that Jiraiya, after all his travels, returns to the Hidden Leaf Village and meets Naruto. They could have a series of adventures together, like exploring some long - forgotten parts of the village or training the new generation of shinobi. Jiraiya could share some new jutsu or ninja wisdom with Naruto, which Naruto can then pass on to his students.
His brother could be an unknown Uzumaki. Since Naruto is from the Uzumaki clan, it would make sense for his brother to be from the same clan. Maybe he was hidden away for some reason, like to protect him from the same threats that Naruto faced as a child.