As much as I love the show, I don't know if this is a hole it can write itself out of. But as I've thought about it, I keep on returning to one potential plotline:
Rosa Diaz gets kicked off the force for police brutality.
Now before I go further, I want to make two things clear.
(1) I adore Rosa Diaz. She's possibly my favorite character on the show. She's a queer icon. Stephanie Beatriz is a treasure.
(2) Rosa Diaz is definitely the main cast character most likely to physically abuse a suspect. Her whole character is based on her being violent, aggressive, and hot-tempered. She literally jokes about committing police brutality in the show's second episode!It's not hard to imagine the scenario. Rosa is chasing a suspect through New York City alleys. She has to jump over dumpsters and garbage, she's hot, sweaty and frustrated. When she finally catches up with the guy at a dead end, she's basically snarling. And so even though he's cornered and not a threat, she takes him down -- hard. Which someone records, and it goes viral.
At this point, the squad divides. Jake, still hopped up on his childish notions on what it means to be a bad-ass cop, backs up his old friend from the academy; while Amy, in a new leadership position and more exposed to political fallout can't bring herself to defend Rosa's actions. Terry is sensitive to police brutality, having recently experienced a racist confrontation that nearly turned violent, and is surprised to learn that this is one area where Holt -- while not exactly approving -- is a man of his generation of cops, thinking that a rough take down of a suspect is business as usual and not worth getting riled up about. Hitchcock and Scully choose opposing sides for arbitrary reasons. Boyle is paralyzed by indecision.
Jake seizes on the notion that if he can prove the suspect really was guilty of a crime, Rosa's actions will be seen as justified. He works the case feverishly until he eventually discovers that the man Rosa injured had some drugs in his apartment -- a triumph, until Amy points out the obvious so what? So what if the guy smoked a few joints? Does that mean he deserved to be abused? Is Jake really going with "he's no angel"?
And so the resolution is not that Rosa is let off the hook, or learns a valuable lesson, or has the squad unite behind her. The resolution is that Rosa is fired from the NYPD (and, I imagine, written off the show).
Does it have to be Rosa? Could it be a random Nine Nine beat cop we had never seen before instead? No. It has to be Rosa, because it has to be someone we care about. The problem of police abuses is misjudged if it's viewed as the product of a few sadists hidden from public view. Those people exist, but the larger issue is that police abuse occurs by men and women who are in other respects normal, likable, courageous -- people who do good things, have friends who care about them and who care about others, people who in other contexts may do good or even heroic deeds. The Florida cop who attacked a peaceful protester, the one with 79 use of force complaints in three years? He also stopped a suicidal woman from jumping off a bridge. I bring this up not as an excuse -- just the opposite. It is to hammer home the gravity of the problem. This is the banality of evil at work; we deceive ourselves if we think it is a problem that is restricted just to some anonymous snarling monsters. We have to get used to the idea that police violence (like all injustices) are perpetrated by people who look familiar to us.
It has to be Rosa because it has to be someone who has already been fully fleshed out as a human, with the full array of human relationships and feelings and sentiments and history that humans carrying with them. It has to be someone we care about. Only that will give the issue the gravity it deserves.