One common theme is self - discovery. When Naruto is alone, he often has time to think about who he is and what he wants. Another theme is inner strength. He has to rely on his own abilities and not depend on others. For example, in many fanfictions, he overcomes great difficulties alone using his own skills.
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.
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.
Maybe an ancient evil ninja who was sealed away long ago but has now been released. This antagonist could have powers that nullify any communication or summoning jutsu, forcing Naruto to fight alone. He would be extremely powerful and know many forgotten and deadly jutsu.
In a 'naruto fights alone fanfiction', it could be that Naruto is on a mission in a foreign land where he has no support. The local ninjas are all against him for some reason. He has to use his Nine - Tails chakra in a more discreet way as he can't afford to go all - out and draw too much attention. He might befriend some local animals or spirits who help him in a more indirect way, like giving him information about the enemies' weaknesses.
One common theme is self - discovery. When Naruto is alone, he has more time to think about who he is. He might question his purpose in life, like whether he's just a tool for fighting or if there's more to him.
Maybe Naruto is alone because all his friends are with their families. He might be reminiscing about his past Christmases, or perhaps he's on a mission during Christmas and feels a bit lonely while seeing the festive cheer around him.
It's popular because it shows a different side of Naruto. We usually see him with his friends, but when he's all alone, it creates a sense of vulnerability that's really interesting to explore.
It could be about Naruto choosing a solitary life after all the battles. Maybe he decides to live alone in the forest to find inner peace away from the chaos of the ninja world.