Saitama's rank in the web comic is sort of unconventional. His strength is off the charts, making it difficult to slot him into a specific rank. You could say he's in a league of his own, far above the typical rankings.
In Da Feng's Watchmen, Xu Qi 'an entered Rank-4 in chapter 1136.
While waiting for the TV series, you can also click on the link below to read the classic original work of "Dafeng Nightwatchman"!
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.
The main character, Fan Xian, had become a ninth-rank in the beginning of the novel," Celebrating Years." He was a transmigrator who possessed the knowledge and skills of modern society. Furthermore, he had received help from his mother, Yu Yin, which gave him a special advantage in this world. Thus, Fan Xian had already reached the ninth-level at the beginning of the story.
Another plot could involve Saitama using his new S - Class status to reform the hero system. He might see all the flaws in it, like how some heroes are more concerned with fame than actually saving people. So, he starts making changes from the inside. He could introduce new training methods or even a new ranking system that focuses more on a hero's selfless deeds rather than just their power level.
Saitama's relationships would be completely upended. Friends like King would no longer be able to approach him casually. And his relationship with the citizens would change from protector to feared being. His new monstrous nature might also attract other villains and monsters, who could either try to follow him or challenge his new - found power.
Genos would probably be even more in awe of Saitama. He already idolizes him, but with Saitama being S - Class, Genos might see him as even more untouchable. He could become more protective of Saitama's new status, trying to fend off any detractors or those who might try to take advantage of Saitama.