In some fanfics, Jiraiya could visit Naruto even during his seclusion. Jiraiya might have some unfinished business or new teachings for Naruto. He could pass on some secret jutsus or give Naruto some insights about his future. Or perhaps some of Naruto's friends from the Hidden Leaf like Shikamaru or Sakura might visit him secretly, bringing news from the village and just showing their support for Naruto during his time away.
The Nine - Tails could be considered as a 'visitor' in a way. Since Naruto is in seclusion, he has more time to interact with the Nine - Tails inside him. Also, it's possible that some of the spirits of the past Hokages might visit Naruto. They have a connection to Naruto, and they might want to guide him during this important time of self - improvement. They could share their experiences and knowledge to help Naruto become a better shinobi.
Sasuke could visit him. Despite their differences in the past, they have a deep bond. Sasuke might come to check on Naruto, or maybe he also wants some time away from the world and thinks Naruto's seclusion place is a good spot.
It's possible that he secluded himself to protect his loved ones. In the Naruto universe, there are always threats. By isolating himself, he could avoid putting others in danger while he became stronger. He may have also been secretly gathering information about his enemies during this time.
His teammates, Sasuke and Sakura, would surely visit him. They are his closest comrades, and they would be worried about his condition.
His best friends Hermione and Ron would surely visit. They might come to help Harry set up new magical experiments or just to relax away from the hustle and bustle of the wizarding world.
His relationships with his friends might have become strained. Since he was in seclusion, he wouldn't have been able to interact with them regularly. They might have felt abandoned and been initially angry or hurt.
There's a chance he'll meet other lost souls, people who are also on a journey of self - discovery. They could be from different villages and have their own reasons for being in the area. Naruto's positive attitude and determination might inspire them, and they could end up traveling together towards Uzu, sharing their stories and skills along the way.
In a Naruto 3 years seclusion fanfic, Naruto could focus on intense training. He might master new jutsus that he didn't have time for before. For example, he could develop a more powerful Rasengan variation. He could also gain better control over his chakra, perhaps even unlocking new levels of his Nine - Tails chakra. This seclusion time would be like a cocoon phase, where he emerges as a much stronger shinobi.
His old mentor, Iruka, might visit. Iruka has always cared about Naruto. He could bring some supplies and words of encouragement. He might also be worried about Naruto's well - being and want to make sure he's okay living in the cave.
He might meet some of the past hosts of the Kyuubi. These past hosts could share their experiences with Naruto, giving him valuable insights on how to handle the power of the Kyuubi. It could be like a gathering of all those who have carried this great burden and power before him.
Naruto might meet his future children. He would be able to see what kind of people they are and what their dreams and aspirations are. It would be an emotional moment for him, seeing his family in the future.