One common theme is the struggle between love and duty. The characters are police officers first, so they have to balance their personal relationships with their responsibilities to the city. For example, they might have to cancel a date because of an emergency call.
I'm not sure specifically without more context, but it could be some of the well - known characters from 'Chicago PD'. Maybe Voight could be involved in some way, or Halstead.
It could be about the romantic relationships that develop among the characters in the 'Chicago PD' series. Maybe it's a story of a cop falling in love with someone in the city, perhaps a civilian or another officer.
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 love story could be between two officers who start off as partners. They constantly have each other's backs during dangerous situations. As time goes by, they realize their feelings for each other are more than just friendship. They might share a special moment after a tough case, like a quiet dinner together, and slowly their relationship blossoms.
One great 'chicago pd fanfic' is 'The Blue Line Bonds'. It delves deep into the relationships between the characters outside of their regular work scenarios. The character development is on point and it gives a fresh perspective on the show's dynamics.
You can start by looking on fanfiction.net. It has a large collection of 'Chicago PD' fanfics, including many focused on Hailey. Just use the search bar and type in 'Chicago PD Hailey' and you'll get a list of relevant stories.