Chicago PD has been playing the long game in season 12. There have been individualized episodes, as always, but the overarching storyline has set up a huge conflict between Chief Deputy Charlie Reid (Shawn Hatosy) and Hank Voight (Jason Beghe).
The latter has effectively been forced to do battle with a mirror version of himself for the future of the IU. It's not looking good, given Reid's authority and the fact that he knows how Voight thinks because he thinks basically the same way. It's a great set up, but one that would have benefited exponentially from the involvement of Hailey Upton (Tracy Spiridakos).
Upton's dark past would've come to light
Upton left in season 11, but there are several reasons why should have elevated the conflict between Reid and Voight. For one, the character has plenty of skeletons in her closet. She killed a criminal who tried to take out Voight, and proceeded to lie about the circumstances in which he disappeared.
When she was nearly found out, she teamed with Voight and then-husband Jay Halstead (Jesse Lee Soffer) to blackmail the agent tasked with learning the truth. Chief Deputy Reid has proven to be such a dangerous force in season 12 because he knows how to dig up dirt on people, and used said dirt to his advantage.
He knows Dante Torres (Benjamin Levy Aguilar) played fast and loose with the rules when it came to dealing with the informant Gloria Perez (Yara Martinez). The young officer slept with Perez, then tried to cover up his sins leading up to her death. One can only imagine the vice grip Reid would have on Upton if she were still part of the IU.
Upton's bond with Voight would be tested
Then there's the Voight and Upton relationship. These two grew extremely close over the years, as evidenced by their intense final scene together in season 11, but the circumstances in which they interacted proved to be too much for Upton. She could no longer contend with the darkness, she needed to get out.
If Upton were still part of the show, then her friendship with Voight would take on fascinating dimensions. She would not only have to contend with her desire to break free of the IU and its morbidity, but with Reid, who exemplifies all of Voight's worst traits and would be able to exploit Upton's unrest due to her past indescretions.
Look, we're happy Upton has moved on. She had an absolutely punishing couple of seasons before she left, and looked genuinely happy as she left the IU and the Windy City behind. It's probably for the best that she doesn't have to contend with Reid, even if, selfishly, we feel like it would have for riveting television.