Protection. Given that it's a show about firefighters, protecting the baby from harm, whether it's from a fire or other dangers in the city, is a big part of the fanfiction. The characters will go to great lengths to keep the baby safe.
There are often fanfictions where a character in Chicago Fire has a surprise baby. In these stories, the drama unfolds as they struggle to balance their dangerous job with new - found fatherhood or motherhood. They might have to deal with the fear of leaving the baby without a parent if something were to happen to them on a call. It adds a whole new layer of emotion to the story.
One common theme is family expansion. For example, a couple from the show might have a baby, and the fic would explore how they balance their work at the firehouse with their new family life. Another theme could be the support system within the firehouse. When a character has a baby, the other firefighters might step up to help, showing the strong bond among them.
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.
Definitely the other firefighters at the firehouse. Casey, for example, could be a great support. He could offer Severide advice on being a father, based on his own experiences or just his general good - natured wisdom. Boden, as the chief, could also play an important role. He might be strict about work - life balance but also show a soft side when it comes to the impending arrival of the baby. And of course, the female firefighters could be there for Shay, like Gabby. They could have girl - talk about pregnancy and motherhood.
One idea could be that Shay and Severide find out they're having a baby unexpectedly. They then have to deal with the reactions of their friends at the firehouse, some of whom are overjoyed while others are worried about how it will affect their work. Another plot could be centered around a health scare during the pregnancy, making them realize how much they truly care for each other and the unborn baby. Maybe they could also have a disagreement over parenting styles even before the baby is born, like Severide being more laid - back while Shay is overly cautious.
A fanfiction might focus on Peter Mills' relationships at the firehouse. For example, his friendships with the other firefighters. He could get into an argument with one of them over a difference in approach to a job, but then they work it out and their bond becomes even stronger.
I'm not sure of specific 'Chicago Fire' baby fanfic titles off the top of my head. But you could also look on Tumblr. There are often fanfic recs on Tumblr pages dedicated to 'Chicago Fire'. Some fanfic might focus on the idea of a character becoming a new parent, which could bring a whole new dynamic to the character relationships within the show.
A great character interaction could be between Severide from Chicago Fire and Voight from Chicago PD. Severide's laid - back but confident attitude could clash or complement Voight's tough and no - nonsense approach. They might start off not seeing eye - eye but then come to respect each other as they work towards a common goal, like solving a crime that is related to a fire incident.