Well, an overpowered MC usually has insane strength, skills, or abilities that make them way ahead of others. They can easily overcome challenges and enemies with little effort.
You might want to check out 'Arcane Ascension'. The main character has OP magic skills and overcomes every challenge with ease. The plot is engaging and the magic system is well-developed.
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.
The manhwa leveling system usually involves characters gaining experience and completing tasks to increase their levels and unlock new abilities or skills.