I don't want to write your book for you but I have a few ideas that you could maybe put into your story
-2 detectives that have been assigned to a simple case but it turns out to be a lot bigger than it should have been. Maybe a few twists in the case
-A corrupt detective is payed to tamper with evidence and the crime scene by a hitman and soon finds out that the defendant is someone that is very close to him...
-An honest detective that is forced into blackmailing the defense lawyer in a case so the verdict that is handed down on the defendant is not guilty
Those are my ideas