A fanfic about Joe Shimamura as a child in the Cyborg 009 universe could explore his early days before becoming a cyborg. Maybe it could show his family life, his dreams and hopes that were cut short when he was transformed. It could also focus on how he interacted with his friends and neighbors, and what kind of a person he was growing up. For example, was he a brave kid from the start or did the events that led to his cyborg transformation shape his courage?
Well, in a Cyborg 009 Joe Shimamura child fanfic, it might depict his first encounters with technology. Since he later becomes a cyborg, perhaps as a child he was already interested in mechanical things. It could show him tinkering with old radios or broken toys, trying to fix them. This would give a backstory to his later acceptance of his cyborg enhancements. And it could also show how he dealt with any hardships in his childhood, like poverty or bullying, which might have influenced his character development into the strong cyborg we know.