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.
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.
The main theme could be about survival. Since they are rock eaters, perhaps it's about how they adapt to their strange diet and the challenges it brings in their environment.
To write a Genos crossover fanfiction, think about the differences and similarities between Genos' world and the other universe. For example, if it's a crossover with a magical world, how will Genos' technology interact with magic? Develop your characters well, not just Genos but also the characters from the other universe. Make sure their interactions are believable. You can also add some humor or drama to make the story more engaging. Maybe Genos tries to learn magic but keeps failing in a comical way. This can add a light - hearted touch to the story.