The city's diverse population has also had an impact. With different ethnic groups, cultures, and social classes coexisting, it gives writers a lot of material. In Chicago fictions, we can see stories about immigrants trying to make a living, or about the clashes and reconciliations between different groups. The city's rich history of labor movements, for example, has also inspired many fictional works that deal with the struggles of the working class.
A crossover could involve a character from Chicago Fire having a personal connection to a case the Chicago PD is working on. Maybe a firefighter's family member is wrongly accused of a crime. The firefighter then teams up with the PD detectives to clear their family member's name. This would bring in elements of loyalty, family, and the different skills of both groups. They could share information and resources to solve the mystery.
One popular theme is the relationships between the characters. For example, romantic relationships that might not be explored as deeply in the show. Another theme is the characters' backstories. Fans like to fill in the gaps and create their own versions of how the characters became who they are.
The 'Chicago real story' could refer to many things. It might be about the city's history, like how it grew from a small settlement to a major metropolis. It could involve stories of the people who built the city, including the immigrants who came and contributed their labor.