His perspective on power might change. In the Harry Potter world, magic is a natural part of life for him. But in RWBY fanfiction, power comes in different forms like semblances and the use of Dust. He would see that there are other ways to be strong and protect people that don't rely solely on spells.
Naruto's perspective could expand in terms of power usage. In the BNHA fanfiction, there are so many unique quirks and how they are used in different situations. He might start to view his own chakra and ninja techniques in a new light. He could think about ways to combine different techniques in a more creative way, similar to how the characters in BNHA use their quirks in combination. He may also become more aware of the importance of public perception, as heroes in BNHA are very much in the public eye, which is different from the ninja world where secrecy is often key.
James Potter's perspective would likely change in several ways. He might gain a new appreciation for how his story has inspired others. He could also become more aware of the parts of his life that people find most interesting. For instance, if fanfiction often focuses on his time at Hogwarts, he might start to think more about those days and how they shaped him. At the same time, he might be a bit dismayed if the fanfiction misrepresents him or his friends. This could make him more determined to set the record straight about certain events.
His perspective could become more open - minded. He'd see different interpretations of battles and characters, which might make him question his own assumptions about combat and relationships in his own universe.
His view of magic could expand. Anime often has very different interpretations of magic and superpowers. He might start to think more broadly about what magic can do and how it can be used.
His view could become more broad. He'd see that magic can be manifested in ways other than what he's used to. For example, the zanpakuto in Bleach have unique powers that are different from any wand magic he knows.
Harry might be amazed at the action - packed scenes in RWBY fanfic. The battles are so different from what he experiences in Hogwarts. He could be both fascinated and a little bit bewildered by the complex relationships between the RWBY characters. He might also wonder about the role of the different academies in RWBY, as it's similar to Hogwarts in some ways but also very different.
Harry Potter's view on heroism might shift. In Fate Stay Night, the heroes have different backstories and goals. After reading the fanfiction, Harry could start to think about how his own heroism is defined not just by his fight against Voldemort but also by his relationships and the choices he makes in general. He might also see that heroes can come from different places and times, not just the wizarding world he knows.
His view could become more open - minded. He'd realize there are other ways to approach the supernatural world. For example, the way the hunters in Supernatural use weapons instead of wands might make him think about alternative forms of magic defense.
Well, it could potentially give him some new ideas. Harry is used to magic in his own world, but the technology - like Dust - in RWBY's world could inspire him. He might start thinking about how to incorporate some of those elements into his own magic, or at least be interested in the different ways power is used in RWBY fanfiction.
His view of magic might expand. In his world, magic is mainly about spells and potions. But after reading HTTYD fanfiction, he would see that magic can also be about the bond between humans and creatures like dragons. This could make him think more about the potential of magic in his own world beyond just what he has been taught at Hogwarts.