A competitive stealing game. You play as two rival thieves guilds in a city inspired by the early Ottoman Empire, with light steampunk/magic elements (just enough to justify whatever game mechanics, because muskets are no fun). You each have a stronghold that the other team cannot initially enter; in between your strongholds is a network of alleyways, open market areas, and guarded buildings. The NPC citizens and guards do have an AI that lets them respond to the players, but its highly predictable and transparent; the other team is the main threat. Your goal is to mug, pickpocket, and heist your way into more funds and then get them back to your stronghold, where half the funds go to you to buy upgrades and half go to the guild. The teams can steal from each other as well as the NPC citizens; killing a player causes them to drop half their loot, stealing without killing gives all of it. If you steal from the same shopkeeper or structure three consecutive times without the other team ever doing it you get a "protection" agreement with them, meaning you lose the ability to steal from them and instead get a steady flow of money at no effort, which you only lose if the other team does steal from them. You can purchase security guards, traps, and the like to prevent this from happening, and also use the same tools to secure your stronghold. The strongholds are protected by mysterious, shadowy door guards that magically prevent the other team from entering, but can be bribed; better entrances cost more money. Once inside the enemy stronghold, you can steal their loot, or destroy their property to force them to pay for repairs; you win when the enemy's guild funds are reduced to 0.
Upgrades you can buy would include:
Knives speed up the process of mugging citizens, and can be thrown at enemy players to incapacitate them briefly. Incapacitated players can be mugged to steal their loot.
Guns, mainly for killing enemy players. Concealed guns all take multiple hits to kill and have at least a little bullet spread; while larger guns are perfectly accurate and some can score one-shot kills, but will get you attacked by guards very quickly if you leave your stronghold with them.
Normally players are very obvious to the other team (although not to the NPCs) but they can buy disguises of the various types of citizens; citizen disguises merely hide your from the other team, while guards allow you to carry large weapons openly, and nobles/bankers/scholars/religious figures make it easier to infiltrate specific types of buildings. In the crowded markets where players are trying to pickpocket everyone, having a disguise might let you pickpocket the other team.
Climbing claws/grappling hooks/various other tools to let you parkour everywhere.