The fanfiction could also take a darker turn. Naruto might start to question his own mortality and the meaning of his life. He could become frustrated with his body's weakness, especially considering how strong he's always been. This could lead to some internal conflict, as he has to deal with the reality of his situation while still holding onto his ninja way.
Perhaps it could explore how the news of Naruto having cancer affects the other characters. For example, Hinata could be determined to find a cure, using her family's resources. The story could also show how Naruto's positive attitude in the face of cancer inspires those around him, like how he still tries to cheer up the village children even when he's going through treatment.
One way could be to show Naruto's struggle with the illness while still trying to protect the village. He might have to train his friends to take over his duties in case he gets too sick.
One way could be to show Percy's internal struggle. He might initially try to hide his illness from his family because of his pride and the distance he had with them before. For example, he could keep going to work at the Ministry, pushing himself until he physically can't anymore. His symptoms could start to show during important meetings or tasks, creating a sense of drama and concern among his colleagues who don't know the real cause.
One way could be through Naruto's admiration for Kisuke's unique skills. Maybe Naruto discovers Kisuke's swordsmanship or his knowledge of spiritual energy. He then starts to study Kisuke's ways, trying to incorporate some of his techniques into his own fighting style. This would lead to Naruto changing his combat approach and even his way of thinking about battles.
In a fanfiction, Naruto could be exposed to a new form of chakra or energy similar to Raiden's electricity - based powers. This exposure could lead him to study Raiden's combat style. For example, Raiden's ability to move at high speeds and his proficiency with a sword could inspire Naruto to incorporate new techniques into his own fighting repertoire, thus making him a Raiden fan.
One way could be through a strange crossover event. Maybe Naruto gets sucked into the Bleach universe and is somehow influenced by the power and allure of the Espada. He could start to adapt their fighting styles, for example, learning from their use of reiatsu and zanpakuto - like abilities. This would change his approach to battles in the Naruto world when he eventually returns.
The soul power could be related to the Nine - Tails. Since the Nine - Tails is such a powerful entity within Naruto, his soul power might be a way to better control or even transform the Nine - Tails' power in a more positive and spiritual way. For example, instead of just using the Nine - Tails' chakra for brute strength, he could use his soul power to make the Nine - Tails' power more harmonious and less destructive.
One way could be by combining Naruto's ninja abilities with Jedi powers. Naruto's chakra control could be related to a Jedi's Force control. For example, his Rasengan could be like a Jedi's unique Force technique. Also, Naruto's strong will, which is similar to a Jedi's determination in the light side of the Force, could play a major role in leading the Jedi Order.
One way this could be developed is by having Naruto's abilities misused or corrupted in the Hellsing universe. Maybe some dark magic or experiment twists his chakra into a force that creates 'monsters'. For example, his shadow clone jutsu could be warped so that the clones turn into uncontrollable, monstrous entities. Another aspect could be his connection to the Nine - Tails. If the Hellsing organization were to try and harness the power of the Nine - Tails, it could go horribly wrong and lead to the creation of new and terrifying monsters.
One way could be through a time - travel or dimension - hopping plot device. Maybe Naruto gets sent to the Mega Man universe and is immediately fascinated by the high - tech world and Mega Man's heroic deeds. He starts learning about the different robots and their functions, and decides to become a fan. Another idea is that some kind of crossover event occurs in Naruto's own world. For example, a strange artifact is discovered that contains knowledge of the Mega Man universe, and Naruto, being the curious ninja he is, delves into it and becomes a fan.
One way could be by having Naruto's godlike powers be the key to breaking a curse on Elsa. For instance, if Elsa is under some sort of dark magic spell, Naruto's positive chakra could counteract it. His ability to sense negative energy could lead him straight to the source of the problem.
One way is to focus on the emotional journey. The reader's fear and despair due to cancer can be contrasted with Fresh Sans' carefree and positive attitude. Their meetings could be like a light in the darkness for the reader.