One fanfiction that's quite popular is 'The Female Saitama's Adventure'. It takes the concept of a female Saitama and puts her in different comical situations in the One Punch Man universe. Another interesting one is 'Female Saitama: A New Hero'. It focuses on how she might interact with the existing heroes and villains in a more character - driven way.
The novelty. Since Saitama is originally male, a female version brings a fresh perspective. It allows for new storylines and character interactions. For example, how would a female Saitama interact with the male - dominated hero society in One Punch Man? It also gives room for exploring different power dynamics and emotional responses.
Well, a lot of One Punch Man fanfictions about heroes seeing Saitama are really interesting. Some focus on their initial shock at his power. For example, Genos is always in awe of Saitama's strength in the original work, and in fanfictions, this can be explored even deeper, like how he tries to understand the source of that power.
If Saitama became a monster in fanfiction, his physical appearance would surely change drastically. His bald head, which is now a symbol of his simplicity and power, might take on a more menacing look. His eyes could turn red, and his muscles might bulge in an even more grotesque way. This transformation could also affect his relationships. Genos, for example, would be in a difficult position. He idolizes Saitama as his master, but if Saitama becomes a monster, Genos might have to fight him to protect the city.
One way is to explore his life before he became a hero. Maybe he had a normal job and was constantly frustrated with the mundanity of it. This could have been the catalyst for his intense training.
A plot where Saitama travels to different dimensions in the One Punch Man universe. He could encounter strange versions of himself or other heroes and villains, leading to humorous and action - packed situations.
One popular storyline could be them going on a training journey together. Genos, being the eager learner, and Saitama, with his overpowered strength, could teach Genos new techniques in unique ways. Another could be them facing a new, extremely powerful villain that challenges even Saitama, and Genos has to step up in unexpected ways to assist. A third one might be a story where they get transported to a different dimension and have to find their way back while dealing with the strange new world and its inhabitants.
First, understand the characters well. Saitama is bored with his power and just wants a good fight, while Boros is a proud and powerful alien seeking a worthy opponent. Then, come up with a unique plot. Don't just rehash their fight from the original. Maybe create a new world or situation for them. For example, a world where all heroes have lost their powers and Saitama and Boros have to work together to save the day. Also, good dialogue is key. Make it sound like how the characters would actually talk. Saitama's simple and straightforward speech and Boros' more grandiose way of speaking.
One common theme is the exploration of Saitama's power in different universes. For example, when he crosses over to a universe with magic, how his physical strength compares to magical abilities.
I also like the scenes where Saitama is just casually hanging out with the heroes, and they're trying to figure him out. He doesn't fit into their idea of what a hero should be, and the awkward yet hilarious interactions that follow are a great part of these fanfictions. For instance, he might be eating a meal with them, and they're all secretly observing him, wondering how someone so ordinary - looking can be so strong.