There could be a stash of valuable chakra - enhancing crystals. These crystals, if found by Naruto, could be used to boost his own chakra reserves or those of his friends. This would be a great advantage in battles against powerful enemies like the Akatsuki. He might also share these crystals with the Hidden Leaf Village to make the whole village stronger.
He may find rare ninja tools. These could be weapons or items with special abilities that no one has seen in centuries. For example, a sword that can cut through any defense or a tool that can enhance one's chakra.
Perhaps he'll uncover a treasure of knowledge in the form of ancient scrolls. These scrolls could contain wisdom about the balance of nature and chakra, something that Naruto, with his strong connection to nature through his Nine - Tails chakra, could use to further his understanding of the world. It could also help him in his quest to bring peace to the ninja world by teaching him new ways to resolve conflicts.
He could uncover secrets about his family. There might be some hidden records in the library that reveal things about his father or mother that he never knew before.
One secret could be the formula for a super - strong chakra - enhancing potion. This would give him an edge in battles against tough opponents.
He might discover long - lost jutsu. Ancient ruins could hold scrolls with forgotten techniques that Naruto could learn and use to become even stronger in his ninja journey.
Well, he might discover a hidden jutsu scroll in the library. It could be a long - lost technique that gives him an edge in battles.
Maybe Naruto discovers modern - day technology in a hidden shinobi village that has been secretly researching it. He could then try to integrate this technology into the ninja world, like using smartphones to communicate during missions instead of traditional methods. This would likely cause a stir among the more traditional ninja.
In a 'Naruto finds a gun' fanfic, one possibility is that Naruto, being curious, might initially mistake it for some strange new weapon from another land. He could try to figure out how it works in his own Naruto - like way, perhaps by trying to infuse chakra into it, which might lead to unexpected and comical results, like the gun shooting out chakra - blasts instead of bullets.
The story in the 'naruto finds a tank fanfic' could also take a more comical turn. Naruto could accidentally set off some alarm on the tank and then start panicking, thinking it's about to attack him. Or he might mistake the tank for a really big and strange animal and start trying to communicate with it in his own Naruto - like way, which would be really funny to read.
In the 'Naruto finds an egg fanfic', Naruto may take the egg to his teacher, like Iruka. They might study it together. It could be an egg of a rare creature. Naruto's sense of adventure would lead him to discover more about the egg, and perhaps it will hatch into a cool animal that becomes his new friend or ally.
He could find the sword in an ancient tomb. This tomb might be filled with traps and puzzles that Naruto has to overcome. Once he gets the sword, he discovers that it glows when near an enemy's weak point. So, he starts using this ability to his advantage in battles.
He could wish for the end of all hatred in the ninja world. This would be a huge step towards achieving true peace. Naruto has always been about bringing people together and this wish would eliminate the root cause of most of the conflicts in his world.