Perhaps an alien race that views beauty in a completely different way from humans. They might see Sweet Mask as an abomination because he is too 'perfect' according to human standards. This would lead to an interesting conflict as Sweet Mask tries to defend his ideals of beauty against these alien invaders.
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.
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.
In a fanfiction, we could see Sweet Mask being secretly a huge fan of Saitama. He could try to get closer to Saitama to learn his secrets of being so powerful. However, Saitama is completely oblivious to Sweet Mask's admiration. This could lead to some comical and interesting interactions between the two characters.
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.
A really popular 'One Punch Man' fanfiction is 'King's True Power Revealed'. It delves into the mystery of King's character and gives him a more significant role. The story is full of twists and turns as it uncovers the secrets behind King's supposed power. Another interesting one is 'The Monster Association Reborn'. This fanfiction reimagines the Monster Association with new and more terrifying monsters, and Saitama has to face them in unexpected ways.
It could be Orochimaru. He's always interested in powerful artifacts, and the mask Naruto gets would be no exception. Orochimaru would likely try to steal the mask from Naruto to add to his collection of powerful items and use its power for his own dark experiments.
It could be someone who also wants the power of the mask. Maybe a rogue ninja who has been researching the mask for a long time and sees Naruto as an obstacle to obtaining it.
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.
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.
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.
The plot could be Metal Bat going on a quest to find a special metal to upgrade his bat. Along the way, he encounters various monsters and other heroes. He might have to compete with other heroes who also want the special metal for their own reasons.