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.
Saitama would probably have an interesting dynamic with Bakugo. Bakugo is brash and confident in his abilities, but Saitama's nonchalant power would likely humble him. Genos could form a partnership with Uraraka. Uraraka's zero - gravity quirk could be useful in combination with Genos' combat skills. They could work together on rescue missions. And Saitama could also have a mentor - like relationship with some of the UA teachers, sharing his unique view on heroism that doesn't rely on quirks but on personal effort.
Goku's ultimate form in the manga is Ultra Instinct. This state grants him enhanced combat abilities and reflexes. It's a breakthrough that showcases his potential and is a key factor in his toughest battles.
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.