I'm currently on my third version of a homebrew adventure path and have run into some idea blocks.
At the beginning of the AP (going from levels 1 to 4~5) the PCs will be in a large city that was the capital city of one part of a now crumbling empire. The new King of this newly formed kingdom has moved to another city and left this large city of about 50,000 to fend for itself.
What would be of great help would be some ideas on size, activities, and organization of the organized crime within the city.
One of the larger organizations is run by a small group of elite persons that are all cultists of Mammon, which will be a source of conflict for the PCs for most of the AP. Other than that, I have very few ideas on size, organization, etc. Thing's that I have few ideas about would be how a high fantasy medieval drug trade would work? How about underground gladiatorial combat?
First, I'd start to ask a few questions about the structure of the city itself - from there, it might become more apparent how to create a believable criminal organization.
1. Who/what is the government? Group of merchants? Local lord who is no longer swearing fealty to the old King? Cabal of wizards? You get the idea.
2. Determining the type and quality of government can then lead to what the local guard and military look like. If this city has been left to fend for itself, it is essentially a city-state. So, who is doing the policing and who is doing the military protection?
3. Is there a alignment tendency to the city? CN or LG or ??. That should also have an impact on what criminals arise from the shadows.
4. The laws (or lack thereof). IS a drug trade illegal? What about slavery? Now that the city is free of the King, perhaps there is some loosening of the law(s). Or the reverse, the people in power (see point 1) are actively getting more authoritarian and many activities, once legal, are no longer. Your original post sort of suggests the city might be LG (if gladiatorial games and drugs are illegal) but should that be the case?
So, set the stage for the city itself and I think the kind of criminals that might arise within it becomes a bit easier to create, and thus, be more believable.
What would be of great help would be some ideas on size, activities, and organization of the organized crime within the city.
Well, to put things in perspective, the Gambino crime family, at its height, had about 24 crews running in the New York City area, and was one of five major criminal gangs. So a city of six million people supported a population of roughly 1000 organized criminals.
Of course, this ignores other, smaller, criminal organizations such as street gangs (think of the Jets and the Sharks) or small, unconnected groups. But it sounds like you're interested in the capo di tutti capi type of organizers and not a group of four or five second-story men.
The advantage of "organized" crime is that they can produce enough force to protect their interests -- if you try to stiff a made man, you're not merely going to be in trouble from him, but from his entire crew, or if necessary from the whole Family. So a small shopkeeper, with maybe two employees, isn't going to cause any trouble, and even a local police unit of 4-5 people isn't going to mess with a twenty-person gang. More than that would probably be unnecessary unless the rest of the city's elites routinely go around with their own private armies of that size.
As to what kind of criminal activity they would engage in,.... anything that makes money, but in general, you make more money from non-violent crime that can be engaged in repeatedly (e.g., gambling) than from violent crime that both attracts attention and causes people to fight back. So I'd start with the basics. A partial list would include
Racketeering, smuggling, extortion, fraud, illegal gambling, money laundering, loansharking, counterfeiting, pornography, bookmaking, prostitution, corruption, bookmaking, labor unions, drug trafficking, fencing, and hijacking.
.... and add other crimes such as kidnapping, robbery, burglary, and murder as needed but mostly to protect their long-term money. Sure, you may get 1000gp kidnapping someone's child, but you're better off getting 1gp/week from twenty different people through a high-end casino.