One possible plot could be that Billy and Max are homeless in a big city. They rely on each other for survival. They might start by finding shelter in an abandoned building. Billy uses his street smarts to scavenge for food, while Max is good at finding warm clothes in the trash. As they struggle, they could meet other homeless people who either help or try to take advantage of them. Eventually, they might come across a kind-hearted person who offers them a chance at a better life.
The fanfic could be about Billy and Max's journey as homeless individuals. They could be constantly on the move, trying to avoid the authorities who might move them from their makeshift camps. Billy, being protective, always tries to keep Max safe. They may have to face harsh weather conditions. Maybe they find a community of other young homeless people and together they form a sort of family, sharing resources and looking out for one another. And through all these hardships, their bond as siblings grows stronger.