There a few assumptions that I used to make this scenario:
- The world is very near to an industrial revolution but not quite there yet
- There is an expansionist Britain-like empire that controls parts of both a USA-like and China-like countries
- This empire has been around for centuries in some form or another
- Factories exist
- Firearms are uncommon but everyone knows someone who has a gun
- Rakshasas, Doppelgangers, Werewolves, and Succubi exist in the campaign world
As for the city itself:
- It's a colonial port founded about 150-200 years ago in not-America.
- All the political power is concentrated in the hands of nobility and their subordinates.
- It refines raw goods from the local territories into ready-to-use materials for factories back in not-Britain
- There is a small community of Dwarves here who maintain the machinery of the factories.
- There is also a community of not-Chinese smaller than the Dwarven community. They leave in a fantasy equivalent of Chinatown
- The not-Chinese get the dirtiest and most dangerous jobs in the factories, port, sewers and outside the city.
Hopefully that seems coherent so far.
Twist #1: The nobles of this area are predominantly Werewolves and have been ever since the city's founding. They control the government and the police. They are careful to control their predatory urges via staged "hunts" outside the city at their country estates. Typical prey includes thieves, criminals of all stripes, those with insurmountable debts, and the occasional stray not-Chinese worker. These victims are rounded up by extremely loyal and secretive police squad who often have at least one Werewolf in the group. The community strongly discourages killing anyone inside the city proper. Offenders quickly find themselves getting shafted in terms of agreements and overruled by superiors. Those who persist in these offenses may find themselves tortured or becoming the prey of a very special hunt. There are approximately 80-120 Werewolves in the city. The werewolves are blissfully unaware of the other shapeshifters in the city.
Twist #2: Fifteen years ago, an elderly martial arts master arrived in not-Chinatown and opened a small school. Preaching edification through mental and physical strength, he quickly became an influential member within the immigrant community. The next year, a wealthy merchant and his wife arrived. They graciously used their funds to improve the local housing and pay for medical treatment that many working families needed. The year after that, a teacher and a lawyer arrived and have been educating the populace. At the same time, more and more violent gangs have sprung up since the master arrived fifteen years ago. The local police don't interfere with not-Chinatown unless violence spills out into the general city but they've been watching anxiously as the gangs become larger and more heavily armed. The five persons mentioned earlier, the master, merchant, wife, teacher, and lawyer, are all Rakshasas and are organizing the gangs. They plan on overthrowing the local government and establishing a pirate enclave here. The Rakshasas know that some of the police and government are Werewolves but vastly underestimate their number. They have a much more accurate estimation of the Doppelgangers' numbers and are currently in a truce with them.
Twist #3: More recently, about five years ago, a number of very vocal dissidents advocating for better working conditions, better wages, shorter hours, and more autonomy have been causing disruptions. There are two types of dissident rhetoric: One is political and says that current working conditions are a violation of legal rights and a thinly-disguised tyranny; the other is religious and preaches in the name of gods of freedom that current working conditions are akin to slavery. Despite frequent police crackdowns, the ringleaders always seem to escape and reappear weeks later to incite more unrest. The truth is that ringleaders are Doppelgangers. The political and religious parties are two allied but independent groups working to sow chaos and an armed revolt in the city. They hope to either draw troops away from the much richer and more strategic ports in the north or to draw garrisons from non-Britain itself and launch a bloody uprising. A stumbling block in the political movement's plan is that they are don't know that Werewolves are in town and they mistakenly believe that the Rakshasas are fellow Doppelgangers. But the religious movement knows better . . .
Twist #4: Three years ago, members of the Doppelgangers' religious movement discovered a secret complex deep in the sewers beneath the site that formerly hosted a stately manor. Within these chambers they found a sizable collection of magical texts and trinkets. But the most important find was that of a summoned Succubus. Summoned fifty years ago by a local wizard to act as lover/mentor, she was bound to a small phylactery warded against the touch of demons. One day, the wizard never returned and so she was unable to leave due to the magics that bound her to the phylactery. Over the many decades she studied, practiced, and improved on all the magic within the complex. Those Doppelgangers freed her and have established a secret cult around her. She has taught them many dark and ancient magics and given them very detailed information on the city. The religious movement/cult knows of numerous secret hideaways in sewers and mausoleums; the identities of most werewolves that were alive fifty years ago and by extension many of those still alive; and thanks the Succubus' divination they know that the mysterious master in not-Chinatown is definitely not a Doppelganger. Unlike the other groups, their motivation has taken on a mystical character. They want to incite the bloody war between the Rakshasa gangs, the Werewolf-controlled government and police, and the Doppelganger-led labor unions then harvest the spiritual energy that the dead and dying will radiate from the comfort of their underground catacombs.
Setting off the tinderbox: The Doppelganger cultists' plan to ignite the powderkeg is fairly straightforward. One of them will impersonate a known Werewolf's Human form then hire a prostitute or invite one of the impersonated Werewolf's friends to a very public place were the police and the public will witness the pair. The Doppelganger will then shapeshift into a hybrid Werewolf form, maul whomever was unfortunate to be its companion for the night, and escape into the sewers. The Werewolves and police will be busy trying to suppress the events and capture the presumed murderer. The Rakshasas will stir up the gangs to fight against monstrous oppressors who are truly monsters; the Doppelganger political movement will do the same. The religious movement will call for a divinely-sanctioned purge of the evil in the city's midst then retreat to the winding warrens below.
After that, even I don't know what happens.