Maybe a former villain who is reborn with a new moral compass. They could start using their villainous skills for good, which would make for an interesting main character as they navigate their new life in the world of One Punch Man, trying to make amends for their past deeds while also fighting new threats.
Another possibility is that the main antagonist is a corrupted hero. This hero, who has turned to the dark side, sees Kombu Infinity as a threat to his new - found power and plans. He uses his knowledge of the hero association and his own powers to try and eliminate Kombu Infinity, all while trying to keep his true identity a secret from the other heroes.
Mumen Rider could be an interesting main character choice. His unwavering sense of justice and his determination despite being a relatively weak hero could be the focus. In a daily life fanfiction, we could see how he trains, his interactions with civilians, and how he looks up to heroes like Saitama even though he may not be on the same power level.
There's a possibility that it's a completely new entity from another dimension or realm. This character with the 'golden name' enters the One Punch Man world with a mission to save lives. They might have abilities that are far beyond what the regular characters in the One Punch Man universe possess. Their golden name could hold the key to some great power or knowledge that is necessary to save the world from a new and greater threat.
The concept could be expanded by giving the punch different properties. For instance, it could have the ability to nullify an enemy's special powers instead of just knocking them out physically.
Genos would be an obvious choice as his student. Genos is always eager to learn from Saitama and improve his skills. Also, some of the lower - ranked heroes like Mumen Rider could be his students. Mumen Rider has a strong sense of justice but lacks in power, and Saitama could teach him some basic self - defense techniques.
A powerful and grotesque - looking villain who specifically targets Sweet Mask because of his obsession with beauty could be a great antagonist. This villain could constantly taunt Sweet Mask about his vanity and try to shatter his perfect image.
How about a group of aliens who have heard about the One Punch Man tournament and are curious to see the power levels on Earth? They come with their own set of powers, like the ability to transform their bodies into different elements. One of them might be able to turn into water, making it difficult for the heroes to land a solid punch. These aliens add a whole new level of chaos and excitement to the tournament as the heroes have to figure out how to fight against such strange and powerful opponents.
One interesting concept could be a character being reborn with a new power set. For example, they might have a unique combination of Saitama's strength and Genos' technological abilities. This could lead to new and exciting battles in the One Punch Man universe.
Maybe Deku could be the main character but with a temporary one - punch quirk. He could gain this quirk through some sort of experiment gone wrong. And he has to learn how to use it while still relying on his original 'One For All' quirk in certain situations. This would add a new layer of complexity to his character development.
The story could start in the middle of a fight. The character, now reborn as Saitama, is in the middle of a battle against a monster. They instinctively use Saitama's one - punch power to defeat the monster and then are left standing there, realizing what has just happened. They then start to think about how they got there and what they should do next in this new life as the world's strongest hero.