Well, he might become an even more formidable wizard - warrior. The katana's swift and deadly nature could add a new dimension to his combat skills. He could use magic to enhance the katana's strikes, like making it cut through magical shields more easily.
In fanfiction, if Harry Potter were to use a katana, it would be a really interesting concept. First of all, he would need to learn how to handle it properly. He could seek the help of some magical beings who are proficient in sword fighting. The katana could also have some magical properties of its own. Maybe it was forged in a magical way in a far - off land. Harry could then combine his knowledge of magic with the skills of using a katana. For example, he could use spells to make the katana invisible when he attacks, or he could use his magic to repair the katana if it gets damaged during a fight.
In a fanfiction scenario where Harry Potter uses a lightsaber, his magical abilities could potentially interact with the lightsaber in interesting ways. Maybe his magic could enhance the lightsaber's power or he could use it in combination with spells for more powerful attacks. For example, he could cast a disarming spell while simultaneously using the lightsaber to keep his opponents at bay. Also, the lightsaber's unique properties could make him a more formidable opponent against Voldemort. It would change the dynamics of his duels and adventures quite a bit.
He might also be a bit confused at times. Some fanfictions deviate a lot from the real events in his life. For example, if there's a fanfiction where he has a completely different upbringing, it would be strange for him to read about. He could also feel a sense of flattery that so many people are interested in his story and want to create their own versions of it.
In a 'Harry Potter uses revolvers' fanfiction, the whole tone of the story might change. The presence of a revolver in a world full of magic would introduce a whole new set of rules. For example, how would magic and bullets interact? Would Harry be able to use magic to enhance the revolver's capabilities? It could also lead to some interesting character development as Harry has to adapt to using a non - magical weapon in a magical world. This could cause conflicts with his friends who are used to the traditional magical ways of fighting.
In some fanfictions, he might find it in an ancient weapon stash hidden in a mythical location. Maybe while on a quest, he stumbles upon a secret chamber where the katana is kept as a powerful relic from an old civilization.
He might be really surprised. Fanfiction often contains different interpretations of the characters and the story. He could be amused by some creative ideas, or perhaps a bit confused if the fanfiction goes too far from the original story he knows.
In a 'Harry Potter in Gotham' fanfiction, Harry might team up with some of Gotham's heroes. For example, he could share his knowledge of magic with Batman. Harry's magical creatures knowledge could also come in handy. He might introduce Thestrals to Gotham as a new form of transport for the heroes during emergencies. Also, his ability to communicate with snakes could potentially help in dealing with some of the more reptilian - like villains in Gotham.
The idea of Harry Potter with a gun is quite interesting. One possibility is that he gets the gun from the muggle world accidentally. He might initially be confused about how to use it but then realizes its potential. This could lead to new alliances in the wizarding world. Some wizards might be intrigued by this new weapon and want to learn more about muggle technology, while others might be afraid and try to confiscate the gun from Harry.
It would be really strange. He might be very confused as fanfiction often contains different interpretations and sometimes wild ideas that are far from his own experiences. He might think it was some kind of strange magic that created these alternate stories about him and his friends.
Well, he could bring his magic skills to the team. Maybe he'd use spells like 'Expelliarmus' to disarm villains before the Avengers could even react. His ability to apparate could also be useful for quick getaways or surprise attacks on enemies.
It would probably be about Naruto having some sort of god - like abilities. He would use a katana in combat, perhaps with skills far beyond what he has in the original series. For example, he could use his godlike chakra to enhance the katana's strikes or even use it to perform special jutsu that are unique to this fanfic concept.