Power and responsibility is likely to be a theme. Naruto gains more power as he progresses in his journey, and he has to learn how to use it responsibly. Also, the theme of perseverance. He faces so many obstacles but never gives up, and this could be further explored in the fanfiction.
A 'naruto a boy's journey fanfiction' could show Naruto's journey in a different light. Maybe it would change some of the events in the original story. For example, it could have Naruto learning new jutsus earlier or having different interactions with his sensei, like Iruka. It could also show his internal struggles more vividly, such as his feelings of loneliness when he was younger.
One possible theme could be displacement. The idea of being uprooted from one's home and becoming a refugee, like a ghost floating without a place to belong.
One possible theme could be family. Christmas is often a time when families come together, and in a story about a boy at Christmas, family bonds could be a central theme.
Another theme might be growth. Through their interactions with the birds, the boy or boys could grow in understanding, compassion, and a sense of responsibility. For example, if they rescue a bird, they learn about taking care of another living being. This could be a step in their personal growth journey.
One possible theme could be friendship. The boy might form deep bonds with other characters in the story, and these friendships help him through tough times.
The main characters would most likely start with Naruto. But we can't forget about his friends from the Hidden Leaf. Shikamaru, with his intelligence, might be involved in strategizing for Naruto's journey. Hinata could also be a main character, given her relationship with Naruto. And there's always the possibility of villains from the past making a comeback or new villains emerging as main antagonists in this second journey.
One possible plotline could be Naruto having some of Beast Boy's shape - shifting abilities in addition to his ninja skills. He could use this to his advantage in battles, for example, turning into animals to surprise his opponents. Another plotline might involve Naruto trying to understand his dual heritage and how it affects his place in both the ninja world and the world where Beast Boy comes from.
Responsibility is surely a major theme. It could be about how the boy learns to take on different kinds of responsibilities as he grows up.
One possible theme is the power of nature in love. The mountains are a big part of the boy's life, and his love could be influenced by the beauty and harshness of the mountain environment.
One possible theme could be resilience. If he faced hardships, his ability to bounce back would be a significant part of his story. Another theme might be friendship, if his friends played an important role in his life.