It depends on various factors. In the fanfiction community, some might find it an interesting and creative exploration of characters, but others may object due to the negative connotations of cheating. However, as long as it adheres to the general rules and ethics of the fanfiction platform, such as not promoting harmful or unethical behavior in a glorifying way, it could be considered acceptable in the realm of creative writing.
One challenge is dealing with the negative perception of cheating. Readers may be put off by the idea and it can be hard to make them sympathetic to the characters involved. Another challenge is staying true to the character of Genos. His personality and values need to be considered when writing him in such a complex and morally ambiguous situation.
Saitama and King can also be a popular pairing. King, despite being seen as a powerful hero, is actually a bit of a coward. Saitama, on the other hand, is extremely powerful but often underestimated. Their interactions can create some really funny and interesting stories, like King trying to pretend he's as strong as Saitama in front of others.
In some saitama x everyone fanfiction, there could be a storyline where Saitama has to train other heroes. Since he's so powerful, he might be able to teach them new techniques or ways of thinking. This could lead to the growth and development of other characters, like Mumen Rider becoming a more capable hero under Saitama's guidance.
One of the things that make 'sonic x saitama fanfic' so appealing is the potential for character development. Sonic, who is used to being the hero in his own world, has to adjust to Saitama's way of doing things. And Saitama, who is often bored with the lack of challenge in his world, finds Sonic's energy and his world's constant threats exciting. This interaction between the two characters allows for new relationships to form, new challenges to be faced, and new adventures to be had, which is what makes these fanfics so engaging for readers.