He might meet an old acquaintance who is also alone. They could share their stories and perhaps find some comfort in each other's company.
Maybe Naruto meets a group of children who are lost or in need of help on Christmas. Naruto, being the kind - hearted ninja he is, would help them out, and in the process, he might find a new sense of purpose during his lonely Christmas.
There's a chance he meets a mysterious stranger who has a unique perspective on Christmas. This person could teach Naruto something new about the holiday spirit, even though he's alone.
He might meet an old sage who has been living in seclusion near the village. The sage could see Naruto's loneliness on his birthday and offer him some ancient ninja knowledge.
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.
He could meet an old enemy who has also changed after the war. This enemy might now be in a similar situation of being alone and they could form an unexpected bond as they understand each other's isolation.
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.
Naruto could meet some young and upcoming Iwa ninja trainees. They might be curious about Naruto and his abilities from Konoha. These trainees could either look up to Naruto or be jealous of his fame, leading to various interactions. Also, there could be some old - timer Iwa ninjas who remember past events involving Konoha and have their own views on Naruto's presence in Iwa.
Naruto might meet Itachi in heaven. Itachi could finally reveal all the secrets and the true intentions behind his actions to Naruto. And they could become true friends without any misunderstandings. Additionally, Naruto could also meet Neji, and they would talk about how the ninja world has changed since Neji's sacrifice.
Naruto could also meet the previous Hokages in their prime. He could learn from them directly and perhaps even influence their decisions. For example, he might be able to convince the First Hokage to handle the Uchiha clan's situation differently, which could have a huge impact on the future (or in this case, the future that Naruto comes from).
Naruto would meet his younger self. This would be a really strange situation. He could also meet all the important shinobi from the past. For example, the First and Second Hokage. He might have a chance to learn from them directly, instead of just hearing about them in stories. Additionally, he could meet some of the characters who were only mentioned in passing in the original story but had a big influence in the past.
There could be new characters created specifically for this fanfiction. Maybe a young Suna shinobi who becomes Naruto's guide in Suna, showing him around the village and introducing him to the local way of life.
Naruto could meet some young Uzumaki ninja - in - training. They would be excited to meet someone as famous as Naruto and might learn a lot from him. Also, there could be some reclusive Uzumaki who have been hiding special abilities that Naruto discovers during his stay.