Often, there is also a sense of the protagonist being an outsider who has to navigate this new world. They might face initial distrust from the natives but gradually prove themselves. In terms of power, there are often power - leveling or power - up sequences where the protagonist gets even stronger over time. For instance, in 'The Rising of the Shield Hero', the main character starts with a somewhat limited power but gradually becomes very powerful through his own efforts and new abilities he acquires.
One factor is the sense of wish - fulfillment. Readers like to imagine themselves in the shoes of a character who can easily overcome challenges. For example, in 'That Time I Got Reincarnated as a Slime', Rimuru's ability to gain new powers so effortlessly is exciting. Another aspect is the ability to change the world around them. An overpowered MC like Momonga in 'Overlord' can reshape the new isekai world according to his will, which is very appealing.
One popular overpowered isekai novel is 'Overlord'. In this story, the main character is transported into a game world and he has extremely powerful abilities right from the start. He has an entire guild of powerful minions under his control, which gives him a huge advantage in the new world.
A really good overpowered MC isekai novel is 'The Rising of the Shield Hero'. Naofumi is initially treated unfairly but as he progresses, he becomes extremely powerful. His shield gives him unique abilities that set him apart from others in the isekai world. Then there's 'Log Horizon'. Shiroe, the main character, has a great intellect which, combined with his in - game skills, makes him overpowered in the world he's isekaied into. Also, 'Konosuba' has an interesting take on an overpowered MC. Kazuma has some unique abilities, and his party members also contribute to his overall 'overpowered' situation in the comical isekai world.
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.
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.
Well, in some fanfictions, Naruto might obtain the overpowered sword through a mysterious inheritance. Maybe it was left by a powerful ninja from the past and Naruto discovers it in a hidden cave or something.
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.