He might meet the spirit of an ancient ninja.
He might meet an old hermit who has been guarding the temple for centuries. The hermit could have valuable knowledge about Naruto's parents or some secret ninja arts.
He might meet an ancient sage who has been sleeping in the ruins for centuries. This sage could have a vast amount of knowledge about chakra and the ninja arts that Naruto could learn from.
Naruto might run into a group of rogue ninjas who are using the bunker as their hideout. This would lead to a confrontation where Naruto has to use his skills to defeat them and protect the village from their potential threats.
He could meet the previous Hokages. Since the library is infinite, there might be records or even some kind of spiritual manifestation of them. They could guide Naruto further in his journey.
Naruto could encounter a group of rogue ninja who are also after the treasures in the Uzumaki temple. Then he'd have to fight them off to protect the temple and its secrets.
He might meet the spirit of an ancient ninja who was the keeper of the library. This spirit could guide Naruto through the library and teach him things that no one else in the present day knows.
He might meet Hinata. She is kind - hearted and would surely be interested in the puppy. She could offer to help Naruto take care of it, like making a little bed for the puppy.
He would most likely meet his father, Minato Namikaze, and his mother, Kushina Uzumaki. These are the two main family members he never really got to know growing up.
In this fanfiction, Naruto might discover powerful and ancient jutsu hidden in the tomb. Maybe he'll face some dangerous traps too.
It could be an ancient Hokage. Maybe one that was lost in history and had some special powers or knowledge that Naruto could use to better protect the village.