One idea could be that Saitama's hair gives him some new, unexpected power. For example, his hair could start to glow and act as a power amplifier, allowing him to defeat enemies even more easily than before. This would be a fun twist on his already overpowered character.
A plot could involve a villain who has the power to control hair and targets Saitama because his hair is so unique. This leads to a battle where Saitama has to figure out how to fight without using his hair as a weapon, which is quite difficult since his every move usually involves his head (and thus his hair) in some way.
A character could be a hair - stylist in the One Punch Man universe who is obsessed with getting to style Saitama's hair. He or she could follow Saitama around, much to his annoyance, offering different hair - styling ideas and thinking that it would enhance Saitama's already overpowered status.
Another fanfiction concept could be that the monsters in the 'Danmachi' dungeon start to mutate in a way similar to how the monsters in 'One Punch Man' are strange and powerful. This would force the adventurers in 'Danmachi' to come up with new strategies, and perhaps some of the heroes from 'One Punch Man' could come to help out, like Genos, who could provide some technological insights on dealing with these new threats.
The plot could revolve around a villain who targets Saitama's hair specifically. This villain might believe that Saitama's power is directly related to his hair and tries to steal it or cut it off. Saitama then has to protect his hair while also dealing with the normal chaos of fighting monsters in the city.
One idea could be a fanfiction where Saitama's hair suddenly gains sentience. It starts to give Saitama advice on how to be more heroic in a sassy way. For example, it might tell him to change his plain hero costume to something more flashy to attract more fans.
One idea could be a crossover where Spider - Man and One - Punch Man team up to fight a common enemy. Maybe a powerful villain from the Spider - Man universe has somehow made its way into the One - Punch Man world and is causing chaos. They could learn from each other's fighting styles. Spider - Man with his acrobatic moves and web - slinging, and One - Punch Man with his overwhelming strength.
Another idea is that Naruto, curious about other powerful beings, uses a jutsu to travel to the One Punch Man universe. There, he would be amazed by Saitama's strength. Naruto could try to learn from Saitama how to be so effortlessly strong, but Saitama would probably just shrug it off and say there's no special trick. They could team up to fight some of the monsters in Saitama's world, with Naruto using his ninjutsu and Saitama just punching things away.
Well, you could write a fanfiction about a new hero who joins the Hero Association and has a unique power that somehow challenges Saitama's world view. For example, a hero who can control time in a very limited way but uses it in the most strategic manner. Or perhaps a story where all the villains team up against the heroes and Saitama has to face an army of powerful foes all at once. It would be interesting to see how he handles that without just one - punching everyone immediately.
Another concept could be a crossover where some of the Titans gain the power of One Punch Man's enemies. For example, a Colossal Titan-like being that has the regenerative abilities of some of the monsters Saitama has fought. The heroes from both universes would then have to team up to defeat this new threat. They would learn from each other's fighting styles and strategies.
Another fanfiction concept could be that the monsters in Rosario Vampire are being hunted by a group similar to the Hero Association in One Punch Man. However, they don't understand Saitama's power when he shows up to help. They think he's just a normal human trying to be a hero, but are quickly amazed by his feats of strength when he effortlessly takes down powerful vampire or werewolf foes.