One possible fanfiction could be where Lindsay is injured during a dangerous undercover operation. Maybe she gets stabbed while trying to infiltrate a criminal gang. Her partner then has to fight to get her to safety in time and she has to battle her injuries while also dealing with the emotional trauma of the situation.
One idea is that Lindsay gets hurt by a booby - trap. She's exploring a crime scene and accidentally triggers a hidden trap. It injures her leg badly. She then has to rely on her colleagues to carry on the investigation while she recovers. This could also lead to some interesting character development as she has to deal with being incapacitated for a while.
She could be hurt while responding to a chemical fire. The chemicals could cause burns or respiratory problems for her. Her team would be desperate to get her the proper medical treatment, and there could be a struggle to find the right antidote or treatment method.
One common plotline is Casey's struggle with his physical recovery. He might have to deal with intense pain and rehabilitation. For example, he could be frustrated with his slow progress in getting back to his firefighting duties. Another plotline could involve the emotional impact on his friends and colleagues. They might feel guilty for not being able to protect him better. Also, there could be a romantic sub - plot where his significant other is especially worried and their relationship is tested during his recovery.
The 'roommates' trope can also be found in Chicago PD gay fanfic. Imagine two characters who end up living together for some reason, and as they share a living space, they start to notice things about each other that they hadn't before, leading to a romantic relationship. And of course, the 'secret relationship' trope is popular. Given the nature of their jobs in the show, two male characters might keep their relationship a secret from their colleagues at first.
Often, you'll see themes like the OC having a special skill or background that helps Jay and the team solve cases. There's also the theme of the OC facing personal struggles while being part of the 'chicago pd' environment. And of course, the theme of friendship evolving into something more between Jay and the OC is quite prevalent. In many fanfics, the OC might be a new recruit, and the story shows how they grow and interact with Jay in that context.
One great 'chicago pd oc fanfiction' story could be the one where an OC (Original Character) is a detective with a mysterious past. They join the team and bring in a new perspective on solving crimes. It adds an interesting dynamic to the existing characters' relationships.
Definitely Severide. He and Casey are close friends, and Severide would be by Casey's side through the whole ordeal. He'd offer his support in practical ways like helping Casey with his physical therapy exercises if he could. Boden, as the chief, would also be very supportive. He'd make sure Casey has all the resources he needs for his recovery, like getting him the best medical advice. And of course, Stella would be there too. She's a part of the firehouse family and would bring her positive energy to cheer Casey up.