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.
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.
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.
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.
One possible plot could be that Naruto goes on a special training journey with Jiraiya again, but this time they encounter a new and powerful enemy that has a connection to Jiraiya's past. Naruto has to learn a new jutsu from Jiraiya to defeat this foe.
One possible plot could be that Naruto is somehow sent back in time and meets his mother Kushina. He gets to know her as a person, not just as the mother he never really knew. They could train together, and Kushina could share some of her unique Uzumaki clan techniques with Naruto.
Well, the plot could involve Naruto and Meliodas teaming up to fight a common enemy. Maybe an evil force that threatens both their worlds. Naruto's ninja skills and Meliodas' demon powers would make for an epic combination in battles.
Well, in this kind of fanfic, it's not so simple to decide who is stronger. Goku's power levels keep increasing exponentially as he reaches new super saiyan forms. He can destroy planets with his power. Naruto, though, has the ability to harness the power of the nine - tailed fox which is also extremely powerful. Moreover, Naruto has his own unique chakra - based attacks. If they were to fight, Goku might start off with an advantage due to his speed and power. But Naruto is a very quick learner and could adapt during the fight. Also, Naruto has his friends and allies that could potentially tip the balance in his favor in a more complex battle scenario.
Well, in such a fanfic, Naruto would be amazed by Goku's super strength and ability to fly. Goku, on the other hand, would be intrigued by Naruto's ninja skills. They could exchange fighting techniques. Maybe Naruto would learn about ki control from Goku, and Goku would pick up some stealth moves from Naruto. And their friendship would likely develop as they face various challenges together.