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.
He has a sense of justice that can be a bit unorthodox at times. In fanfic, this might lead to him bending the rules a little to get the bad guys. But he always has the greater good in mind. He also has a lot of emotional depth, which can be explored when he's dealing with personal losses or when his past traumas are brought to the surface.
One possible theme could be survival. If they are literally in a cage, they have to find a way to survive against whatever threats are present. Another theme could be trust. Voight and Benson come from different shows ('Chicago PD' and 'Law & Order: SVU' respectively), so the fic could explore how they build trust in this extreme situation.
There are quite a few fanfics out there. One good place to start looking is on fanfiction.net. You might find some stories that really explore the relationship between Voight and his daughter in unique ways, like dealing with the challenges of his job and how it affects their family dynamic.
It could be about a high - stakes situation where Voight and Benson are trapped in a cage. Maybe they're held captive by some criminals they've been chasing, and the fic explores how they use their wits and skills to escape. It might also show their relationship development during this intense situation.
It's interesting because it gives a different view of the Chicago PD world. We see the cases and the police work, but when you focus on Voight's daughter, you get to see how the events in the show impact a family member. It could be about her dealing with the secrecy of her father's job or the loyalty within the police force that her father is a part of. There are so many angles to explore.
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 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.