Another possibility is a character who has just found out that their long - lost dad was a great ninja. This new knowledge makes them determined to find out more about their dad and walk the same path. They could be from a different village, and the discovery of their dad's connection to the Naruto - related ninja world makes them want to explore that part of their heritage.
It could also be a character whose dad was a minor but respected ninja in the village. Maybe a ninja who was part of a special unit during a war and the son or daughter wants to bring back the glory of that unit by following their dad's example.
It could be Naruto's son, if we're thinking in the context of Naruto having a family in the future. He might want to be as great a ninja as his dad and follow in his footsteps.
The plot might involve the main character in the fanfiction facing challenges similar to those their dad faced. It could be about dealing with powerful enemies or making tough moral choices. Say their dad had to choose between loyalty to the village and a personal friendship once. Now the character is in a similar situation and has to decide how to act, all while trying to live up to their dad's reputation.
It could be a vengeful spirit. Maybe someone who had a bad experience during Christmas in the past and now haunts the place, leaving those footsteps as a sign of their presence.
It could also be a young apprentice of the shopkeeper. This character could be learning the ropes of running the shop while also having dreams of becoming a great ninja. They would be in a position to interact with both the regular customers of the shop and the more famous ninjas, which could lead to all sorts of interesting storylines.
It could be Naruto's son or daughter. They would be a natural choice as they inherit Naruto's bloodline.
A new character could be the main character in a 'naruto ambassador fanfiction'. For example, a young ninja who was inspired by Naruto's story and trained hard to be like him. This new character could be given the role of ambassador to show how Naruto's influence has spread even to the newest generation of ninjas, and they could bring a fresh perspective to the diplomatic table.
It could be an original character created specifically for the fanfiction. Maybe a rogue ninja with a vendetta against the Hidden Leaf Village.
It could be Megaman himself. He could be transported to the Naruto world and have to adapt to the new environment while using his skills to make a difference. His sense of justice would fit well in the Naruto universe.
Naruto himself could be the main character. Imagine Naruto getting his hands on a special pistol - like weapon and having to learn how to use it while still relying on his normal ninja skills. It would be a new challenge for him and could lead to some great character development as he tries to master this different form of combat.
Naruto himself could still be the main character. He might be faced with a situation where he has to make difficult choices to prevent carnage, and in the process, his character could develop in new and unexpected ways, like having to use more ruthless tactics.
Rock Lee could be a great main character. He's known for his physical strength and determination. With the Kabutowari, he could develop new fighting styles that combine his taijutsu skills with the power of the weapon. For example, he could use it to break through an opponent's defenses during a high - speed taijutsu battle.