He could meet some sort of powerful demon lords in Tartarus. These demon lords might be initially hostile but Naruto's determination and good heart could change their minds. He might also meet lost souls who are trapped there and try to help them find peace.
Naruto might encounter strange, otherworldly creatures. For example, there could be shape - shifting entities that are curious about Naruto's ninja abilities. There may also be ancient guardians of Tartarus who have been there for ages and have a lot of knowledge about the place that Naruto could learn from.
In a 'naruto hangs out in tartarus fanfic', perhaps Naruto is sent there on a mission. Once in Tartarus, he discovers that his usual ninja jutsu work differently. He starts to explore and finds a group of outcasts. Naruto, being Naruto, decides to help them. He might teach them some of his ninja ways in return for knowledge about Tartarus, like how to find food and shelter in this strange place.
He might meet some lone ninjas who live in the wild. These ninjas could have unique skills or knowledge that Naruto can learn from.
Naruto might encounter future versions of the kages from different villages. These kages could have different leadership styles influenced by the development of the future world. He may also run into new characters who have emerged as heroes or villains in this new era.
Naruto might encounter a group of rogue ninjas. At first, they seem like enemies, but as he gets to know them, he realizes they have their own reasons for being outcasts. They could teach Naruto some unorthodox fighting styles that are not taught in the Leaf Village. Also, there could be a mysterious traveler who has knowledge about Naruto's family that even he doesn't know. This traveler could be the key to unlocking some hidden power within Naruto.
He could meet Konan. Since Konan is an important figure in Amegakure, Naruto might have some interesting interactions with her. They could exchange ideas about peace and the future of the ninja world.
There could be some old and wise shinobi in Suna that Naruto encounters. These shinobi could have stories about the history of Suna and its relations with other villages. Meeting them would give Naruto a broader perspective on the ninja world. Also, he might meet some civilians in Suna who are affected by the village's politics and shinobi activities. Interacting with them would make Naruto more aware of the real - life impact of being a shinobi in Suna.
Naruto might meet some young and talented Kumo ninjas. They could be eager to test their skills against Naruto or learn from him. Maybe there are some unique characters like a Kumo ninja with a special bloodline limit that Naruto has never seen before.
He could meet a lost shinobi from a different village. This shinobi might have been stranded there for a long time and has some valuable knowledge or skills that Naruto could learn from.
He could meet other rogue ninjas. These are ninjas who have left their villages for various reasons, and they might be on a similar journey as Naruto. They could either help him or try to take advantage of his situation.
He could meet a local superhero. They might initially clash because they have different ways of doing things, but then they realize they can learn from each other. For example, Naruto could teach the superhero about chakra control and the superhero could teach Naruto about modern technology and fighting strategies used in America.