One idea could be a joint emergency situation where a building fire is also a crime scene. Maybe the arsonist is still on the premises and the teams from both Chicago PD and Chicago Fire have to work together. The PD has to search for the suspect among the chaos and the firemen have to put out the fire while also trying not to destroy evidence.
A character from Chicago PD could be trapped in a burning building during an investigation. The Chicago Fire department comes to rescue, and during the rescue, they discover some clues related to the case the PD character was working on. This could lead to a closer collaboration between the two departments in the fanfiction, with the characters from both shows sharing their expertise.
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 idea could be a crossover with another police drama, like 'Law & Order'. The characters from Chicago PD could team up with the detectives from 'Law & Order' to solve a case that spans both cities. Another idea is a crossover with a superhero show. Maybe a superhero comes to Chicago and the PD has to work with or around this new element. For example, if there was a crossover with 'The Flash', the fast - running hero could help the PD catch a super - fast criminal that they couldn't handle on their own.
Maybe there's a serial killer on the loose who targets victims in both Chicago and New York. The Chicago PD starts noticing a pattern of killings in their city, and SVU has been dealing with similar cases in New York. They come together to compare notes. The killer seems to be targeting young women who are involved in marginalized communities, whether it's through drugs in Chicago or prostitution in New York. The combined forces of both teams are needed to stop the killer before more lives are lost.
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.
Voight from Chicago PD would be really interesting. He has a strong personality and his no - nonsense attitude would contrast well with the more team - oriented and often more laid - back firefighters. His detective skills could also be crucial in solving a crime that intersects with a fire - related situation.
Another idea is that Erin is hurt during a hostage situation. She might be trying to negotiate with the hostage - taker when things go wrong. The hostage - taker could attack her, leaving her with some serious wounds. Her colleagues would then have to take over the negotiation and also ensure Erin gets medical help as soon as possible.
One idea could be a story about a new recruit who has a hidden past as a firefighter in a different city and has to prove themselves to the Chicago Fire team. They might face challenges like adjusting to the different procedures in Chicago while also dealing with personal issues from their past.
Peter Mills could be involved in a time - travel related fanfiction. He somehow gets sent back in time to an era when firefighting was very different. He has to use his modern knowledge to deal with the challenges of that time while also trying to find a way back to his own time.
A plot could revolve around Brett trying to hide her pregnancy at first because she's worried about how it will impact her career. As the story progresses, she realizes she needs the support of her friends and co - workers. There could also be a sub - plot about her partner's reaction and how they prepare for the new addition to their family.
A fanfic could focus on Voight's backstory. Maybe he has an old case that resurfaces, and he has to deal with the ghosts of his past while still leading his team. It could show his vulnerability and determination in the face of personal and professional difficulties.