A really cool 'One Punch Man Saitama Crossover Fanfiction' is the one that crosses him over with the My Hero Academia universe. Saitama in a world full of quirks, and how his lack of any 'quirk' but extreme physical strength is perceived by the heroes and villains there. It's a unique take on both universes and has some really funny moments as Saitama tries to fit into the MHA society.
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.
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.
The 'One Punch Man Saitama Story' is quite interesting. Saitama is a unique hero. His physical appearance is rather ordinary, but his power is extraordinary. He trained so hard that he lost all his hair and gained the ability to defeat enemies with one punch. In the story, he has to deal with the hero association which has its own rules and politics. Also, he constantly encounters new threats to the city, yet his way of dealing with them is always the same - one punch. However, his interactions with other characters, like his comical exchanges with Genos, add a lot of charm to the story.
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.
One good fanfiction could be 'Saitama's Sister: The Hidden Power'. It explores the idea of Saitama having a sister with her own set of powers. In this story, she tries to keep up with her brother's reputation while also finding her own place in the hero world.
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 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.
For a good character interaction between Garou and Saitama, start with their fundamental differences. Saitama is a hero who is bored with the lack of challenge, while Garou hates heroes. Have them start with a fight, but make it a comical one where Saitama's nonchalance annoys Garou to no end. This can lead to some great dialogue exchanges where Garou tries to taunt Saitama into taking him seriously, but Saitama just doesn't care.
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.