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 common theme is the exploration of an unlikely relationship. Since Saitama is very powerful and Genos is his disciple, in a female Genos x Saitama scenario, it often involves how they navigate their feelings while maintaining their roles in the hero - disciple dynamic.
First, consider the role the OC will play. If it's a partner for Genos, think about their combat abilities. For example, if Genos is mainly a physical fighter, the OC could be a support with long - range attacks or healing powers.
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.
How about Alucard from Hellsing? He is an extremely powerful immortal. In a crossover with Genos, Alucard's centuries - old experiences and his complex relationship with humanity could play off well against Genos' more recent creation as a cyborg. They might start off as adversaries, but then find common ground in their fight against a greater evil. Genos could learn a lot about the nature of immortality from Alucard, who has seen so much over the years.