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.
In this fanfiction, Naruto could be visited by the spirit of his father, Minato. Minato could offer some words of wisdom and comfort to Naruto on his birthday. They might have a heart - to - heart conversation about Naruto's growth as a ninja and his future in the village.
This is a really dark concept. In fanfiction, such a scenario might be written to explore extreme emotional distress. Maybe the author wants to show how much pressure Naruto has endured secretly, and on his birthday, which is supposed to be a happy day, it all becomes too much to bear. However, in the original Naruto story, Naruto is a character full of hope and perseverance, so this is a very different take.
It could be Sasuke. Maybe Sasuke is in a bad mood on Naruto's birthday and rejects Naruto's invitation to hang out or something. Since they have a complex relationship, Sasuke's rejection would really hit Naruto hard.
In this kind of fanfiction, Naruto might feel extremely sad and lonely on his birthday. Maybe his friends forget his birthday, or he is rejected from a special event or by someone he cares about. It could lead to him reflecting on his relationships and his place in the village.
To make it stand out, focus on the emotions. Show Naruto's genuine surprise and joy at the celebration. You can also include some rare characters in the celebration, like the old Hokages coming back as spirits to wish him well.
Iruka would be inconsolable. He was like a father figure to Naruto in the early days. His death on his birthday would bring back all the memories of Naruto's growth and how far he had come. The village as a whole would go into mourning. Naruto was the hero who saved them multiple times. There would be a sense of hopelessness among the villagers, and they would struggle to find a new source of inspiration without Naruto around.
In many fanfictions, Naruto might start by training harder. He throws himself into his ninja training as a way to distract himself from the loneliness. It gives him a sense of purpose and also helps him become stronger.
It could also be related to his past traumas resurfacing all at once. Naruto has had a difficult childhood with being ostracized. If on his birthday, memories of all those painful times come flooding back and combine with a current difficult situation, like a failed relationship or a broken friendship, it could push him to the edge and lead to his suicide in the fanfiction.
Perhaps they would explore their family secrets. Since Naruto has always been in the dark about some aspects of his family, with just the two of them, they could start looking into old family mementos or stories that might lead to uncovering hidden truths.