Building a city watch campaign. What would make interresting NPC's?

[Concrete]

I’m building a pathfinder campaign at the moment, in which the PC’s are meant to work as pretty much an auxiliary unit of the city guard of a large, fantasy city.
Right now, I’m working on the NPC’s that they will be interacting with, and what classes could have what positions in both law enforcement, and criminal operations. I found it pretty interesting, so I wondered what you could come up with.
Here are some that I’ve come up with.

Alchemist.
Works both as a criminal flooding the streets with dangerous experimental drugs, or as a “Q” character, supplying the PC’s with alchemical items before operations.

A low level cleric on loan from the temple of Pharasma, serving as coroner and to a degree, medic. Could mechanically work as a means to get rid of status ailments between missions, and I’m thinking of having here present an adventure hook about a serial killer who disfigures/corrupts bodies to such a degree that her spells can no longer communicate with them.

Barbarian.
Brutish muscle working for a local crime lord. Alternatively, a crime lord that is known for losing his temper more often that is healthy.

I have fiddled with other concepts, like special task forces for the different areas of the city.
For example, a unit that patrols the sewers keeping out the criminal element and minor infestations of oozes and other critters. This unit would consist mostly of creatures with night or low-light vision, for example half-orcs, who would be rather displeased with their work environment.
These feel a bit too involved, though. I mean, what would be the point of sending adventurers down the sewers if they already have these guys to do it? I have to give the PC’s center stage, after all, or what's the point?

Does anyone else have any concepts I could borrow for the entertainment of my precious players? Or are there characters from other campaigns or modules that I’m not aware off that I could buy and rip off? Ideas for plot hooks and interesting crimes would be interesting as well, as would books or other works of fiction that could work as inspiration. (I’ve already stolen quite a bit of the concept from the Watch series by Pratchett.)

*Edit: Ideas about how the presence of monsters, magic, and different races can lead to new and interesting challenges for crimminals, watchmen and normal people in the street are welcome as well

[Red Fel]

Does anyone else have any concepts I could borrow for the entertainment of my precious players? Or are there characters from other campaigns or modules that I’m not aware off that I could buy and rip off? Ideas for plot hooks and interesting crimes would be interesting as well, as would books or other works of fiction that could work as inspiration. (I’ve already stolen quite a bit of the concept from the Night Watch series by Pratchett.)

My advice? Populate the world. Criminals and cops aren't the only ones on the mean streets. Read some classic noir fiction, watch some detective films. Here are a few quickies.
The grunt with a hidden talent. This guy is a low-level grunt for one of the recurring criminals. He has a hidden passion, something embarrassing, perhaps knitting, or singing, or something like that. The PCs may seek to blackmail him with their discovery of his passion, or they may seek to tempt him with offers relating to it (e.g. a chance to audition for a talent scout, some rare fabrics or knitting tools, etc.), in efforts to get around him.
The necessary evil. A moderate-ranked criminal with aspirations for kingpin-ship. He's ruthless, but honorable, and willing to work with the PCs - if not the entire city watch - to take out his rivals. A city with him at the top of the criminal hierarchy is technically a safer one, but whether the PCs are willing to work with a confessed villain is an entirely different question.
The information broker. He's a smug, smirking son of a snake, and he knows it. This is the guy you go to when you absolutely have to know something. He charges high rates - not always money - but always provides exactly what you ask for. And if, at some point, he is desperate, he's a powerful person to have in debt to the PCs.
The obstructing bureaucrat. Maybe he's a local magistrate who's secretly in crime's back pocket. Maybe he's a severe and regimented watch captain with a chip on his shoulder for any activities that aren't strictly by the book. But he's the guy who will frequently be in the way, forcing the PCs to come up with alternative plans while avoiding his watchful gaze. Alternatively, proving to him that they really are good cops, or freeing him from the influence of his criminal blackmailers, may turn him into a fiercely loyal and powerful ally.
The crackpot. He's harmless, drunk, and brain-fried. This makes him invisible. And while nine out of ten things to leave his mouth alternate between lies and insanity, every now and then he has seen something that could turn the investigation upside-down. If he can remember it.
The barkeep. This guy runs the unofficial home base. He doesn't talk much, but he listens, and that means he hears things. Also, ale and steak.
A great city watch campaign is like any crime story; the city itself needs to be a character. You do that by populating it with engaging characters. By having a proper gallery of people with whom to interact, the PCs have options as to how they go about their work. Do they break down doors? Do they buy information? Do they hire some urchins to act as spies? More NPCs - and memorable ones, so the players can keep track - means more options. And not just cops and crooks, but ordinary, entertaining people.

[Geddy2112]

The most dangerous threat to any city is a roving band of adventuring murderhobos. They will come in, selling items plucked from corpses and graves, buy all the good magical swag in town, go to the bar, start a fight, and burn it to the ground. All joking aside, adventuring parties usually bring trouble to cities, or go looking for it when they arrive. Have at least one adventuring party themed session.

Make the PC's the special task force-they do all the stuff the rank and file would fail at, and that the few NPC leaders and local heroes can't be sent to do(as they are spread too thin or whatever).

The city should have some big event or events-festivals, political elections, natural disaster, unnatural disaster. Everything that would happen in a real world city that first response types get involved in.

Between the law and criminals, are going to be a ton of people on neither side, with their own adventures and destines. Don't exclude this group, as they will be the majority, if not 90%+ of the city population.

For adventure hooks, just start watching crime drama/mystery shows and plug and play as you see fit.

[Genth]

I always enjoy the trope of "Those Two Guys" (http://tvtropes.org/pmwiki/pmwiki.php/Main/ThoseTwoGuys), the (usually male) pair who provide commentary and comedy. Think Nobbs and Colon from Discworld.

[Ninjaxenomorph]

I had an idea for a Cait Sith that runs a smuggling ring. Using cats.

[TheYell]

Joseph Wambaugh writes good cop fiction. I would also read Dashiell Hammet's RED HARVEST for a story of a town that earned the name Poisonville. Damon Runyon stories are full of urban color. if you go on Youtube and look up the old Kolchak Night Stalker series for a look at supernatural urban adventure -- the Zombie is a good one to start with. Mentioning Hammett reminds me that the Maltese Falcon works as the story of a mysterious artifact pursued by different chaotic evil creatures and one chaotic neutral sleuth

[Coidzor]

A bard/skald and a troupe of warriors and barbarians that have picked up on the popularity of bar fights and are basically inventing professional wrestling. You can thank the Late Terry Pratchett for the seed of that, what with his Mended Drum bar becoming so popular for its fights that they became choreographed.

An adventurer who has decided to spend his downtime taking on PI work instead of just boozing and whoring his excess gold between adventures. He might even be a real master inquisitive. Or he could be a charlatan who uses his abilities to manufacture work for himself so he gets even more money.

An adventurer who has decided that she's a goddamned superhero and starts kicking the anthill, disrupting the current balance of power between crimelords and the like, setting them off balance as well as upsetting respect for legitimate authority because their immense personal power means they're getting away with it so far and flouting the city's authority. Of course, they're an adventurer, so they're not doing this with a whole lot of finesse. There's tons of things that can be done with either interacting with them, the divided reactions of the guard and populace and criminal element, or dealing with the aftermath of them blowing through town, kicking ass and taking names and dropping a whole bunch of wounded or dead people off at the feet of the local constabulary for a few days and then vanishing for weeks or months off adventuring.

Hedonistic nobles are always fun. Or just have Hedonism Bot have a cameo/expy.

Urchins are always good as potential eyes and ears or people under one's wing what provide plot hooks in a variety of ways.

A vicar or other religious figure that's active at the community level, both amongst his flock and in a general burough might be worth exploring, especially if you have a divine PC and they share a religion or are part of religions that are foils of one another.

A known wannabe cultist who just can't manage to get accepted by or in many cases even find the actual cults that are operating in the city, but could reveal clues due to having a peculiar aptitude for knowing the opposite of what's actually going on amongst the ebon brother and sisterhoods of the city and undercity.

Could look at Echo Bazaar/Fallen London for some inspiration, though it's more gothic

[TheYell]

One thing you might try is have specialty squads at hand to crack the undead, or sewer oozes, or rogue wizards, or evil cults. They're crack high level experts and the Book says to fall back and let them do it...only it costs your party 100xp to make the call for help. so they have a strong urge to break the rules and solve it themselves, and then dodge the inquisitors from Internal Affairs

[DrMotives]

You should also read some Discworld novels. Besides being a series I'd recommend to almost anyone, a lot of them are about Vimes of the city watch, and are full of plots & characters you can use for inspiration. How the city watch deals with (& hires) trolls, dwarves, gargoyles, vampires, as well as normal human constables & criminals.