Maybe it has the ability to absorb chakra. So when Naruto is in a fight against an opponent with a large amount of chakra, he can use the sword to drain some of that chakra, weakening the enemy while strengthening himself. The sword could then store that chakra for later use, like to boost Naruto's own attacks.
It could be a sword that can cut through any substance, like the Susanoo easily. Just imagine Naruto swinging it and slicing through powerful jutsu defenses as if they were paper.
Another aspect is prior knowledge or experience. In some light novels, the MC is reincarnated or transferred from another world. Like in 'Konosuba', Kazuma has knowledge from his previous life which he uses to his advantage in the new world, and this knowledge combined with his unique skills makes him overpowered in certain situations.
The hero might be overpowered due to some special abilities or powers he was born with or acquired. For example, he could have super strength, magic powers, or extraordinary intelligence. And he is overly cautious perhaps because he has experienced great losses in the past when being reckless. So now, even with great power, he still thinks twice before acting to avoid repeating the same mistakes.
It could be that the nature of the world in the light novel is very unpredictable. There could be hidden enemies or powerful forces that even an overpowered hero would be wary of. So, in order to protect themselves and those they care about, they choose to be cautious. For example, there might be ancient curses or prophecies that target powerful individuals like the hero. And the hero, being aware of these potential threats, acts with caution.
Well, it really depends on the specific fanfiction. Some fanfictions do make Ichigo overpowered. They might do this to explore different storylines or just for the fun of it. For example, in some, he could have a sudden boost in his reiatsu that makes him almost unbeatable. But not all fanfictions take this route. There are also those that focus on his character development in a more realistic way within the context of the Bleach universe and don't make him overly powerful.
In 'deku meets future deku fanfiction', perhaps the future Deku warns the present one about upcoming threats. He could tell him about villains he'll face in the future and how to defeat them. This would give the present Deku an edge in his battles and also in his growth as a hero.
Some popular overpowered MC (main character) novels include 'Solo Leveling'. In this novel, the MC starts off weak but gradually becomes extremely powerful with unique abilities. Another one could be 'Overlord', where the MC is an incredibly strong character in a game - like world. And 'The Legendary Mechanic' also features a very overpowered MC with great skills and development in a sci - fi setting.
One of the overpowered Chinese novels is 'Martial World'. The protagonist in it has amazing growth and power - up systems, with constantly evolving martial arts skills and the ability to defeat opponents far stronger than himself at times. It's full of exciting battles and a richly imagined martial arts world.
Immortality combined with rapid regeneration is a very overpowered ability. A character with this ability can never die no matter how severe the injury. They can be blown to bits, and still, they will come back together in an instant. In many fantasy novels, such characters become almost invincible as they can keep on fighting indefinitely, and their opponents have no hope of truly defeating them.