Chicago Fire kicked off the One Chicago franchise in 2012, and since then, NBC has aired three spinoffs, but they should add another one to the list revolving around Matt Casey (Jesse Spencer) and Sylvie Brett (Kara Killmer). Spencer was a part of the original main cast of Chicago Fire in 2012 and was a series regular until he exited the procedural drama in season 10. Meanwhile, Killmer joined the show during the season 3 premiere and was a main cast member until she left in season 12. Given how their stories ended, a spinoff starring Casey and Brett is feasible.
https://screenrant.com/chicago-fire-spinoff-casey-brett-perfect-leads/