Just saw A Working Man, which made me think about Criminal underworld, and as I am home brewing a background for a potential group one day, why not talk about it and see what our thoughts are.

Also how we could shape it for our tables with different age groups and maturity levels (neither are the same)

The Meat Space equivalent to a Thieves guild would be organized crime such as street gangs or the mafia. So Bloods and Hells Angels on one end, Tongs and Russian Mafia on the other.  This is a wide margin of petty thugs on the street, and men in Business suits running the show.

However while in working with thieves guilds we think of theft and little else, like with Skyrim, but also real world cases such as privateering and scams, and getting a heated protest riled up so they can use the chaos to do some smash and grab looting. However most real world criminal organizations deal with other crimes from ones we wonder how this includes smuggling (such as with drugs from the usual list of ones probably not allowed to talk about, to Prohibition era with booze). Human trafficking (one of the great evils out there). Protection Rackets where they force people to pay them to "protect" their businesses, often from the extortionist's own organization. And of course violence, such as calling in hits (assassination) and property damage to get a message across. Violence is also how they deal with those that make their business hard to deal with, be it rivals, rats inside the organization, community organizations trying to improve things peacefully, and law enforcement. 

When dealing with these subjects I would look at the following points.

1. How comfortable are you in covering some crimes. Some things are just uncomeatable, and somethings well others are just Tuesday. A number of us are ok with hundreds of innocents being harmed in a movie, but the moment the dog is hurt or killed we want someone go John Wick on the baddy. Alot of these subjects can be quite heavy, and you might not want to run or play in  game that has them. So, you go to a level at which you are.

2. Age and Maturity level of player. Pirates of the Caribbean: At World's End and Muppet Tresure Island are two Disney films that involve pirates, but one deserved to have  G rating and one deserved a PG-13 (I'll let you decide which is which). The same applies to your game. There are people mature enough to handle PoC level content, and people that can only handle the Muppet version. 

Here are the Thieves Guild's I am working on.

In the one I am thinking of, the setting is some time after a world war level event were a war against a powerful Litch and their undead army ravaged at least one continent, entire kingdoms shattered, and people of all walks of life are piling in the still standing kingdoms as refugees. The Main City this campaign is set around (Crime City until I get a better name) has 5 such Criminal organizations that take advantage of this.

1. A group of Privateers and slavers, they get in all manner of goods, namely booze that is banned, but also do their fair share of slavery and acting as coyotes (people who smuggle people across nation's border's for a profit). The run the ports. 

2. A Pleasure Den run by a Tiger Headed Rakshasa that masquerades as a noble from an exotic land, runs major casinos near the border of the poorer districts, and an open secret about places of ill repute. Those that cannot pay their debts and often sold into slaver, sent to a house of pleasure, or thrown into the arena in the Junkyard to pay off their debts. If he sees a highly valuble mark, he might stack everything against them, namely rigging the game. 

3. Rat Thieves. The one I was originally going to call thieves guild but may need a new name. Rat guild for now as they are a secretive group of thieve that do most of the stealing and do not like interlopers in their city (cuts on business). Run by 13 Wererats with elements of different classes (Theif + Warlock Theif + Fighter and so on). Yes this is a Skaven reference. 

4. Assassin's Guild: The most professional of the group, they handle assassinations that need finesse and skill. Lead by a Snow Lepard Rakshasa, she is the Ex of the Pleasure Den leader and they are not Amical in the least to the point that if either organization  is taken out, they might send the heroes chocolates, wine, a bouquet of flowers and a thank you card. She also has the highest standards of operation and forces those in her organization to keep feuds just business, and never let personal vendettas get in the way of it.

5. Yuan-Ti Drug Dens. Looking for easy slaves for sacrifices, they bring in the drugs that fuels their dens vice.

All 5 work together as they supply each other with goods be it ill gotten gain, slaves, mind altering substances, contracts, easy marks, potential associates, and protection. 

When it comes to taboo subjects or explicit content it is always best to bring these things up at a session zero, ask what sorts of things the players are comfortable with and which things they wouldn't be. Take notes and plan accordingly.

However while in working with thieves guilds we think of theft and little else, ...

D&D showed up after the Godfather Movies.  The whole crime noir movies of the decades before.   Not sure where you got that bit of logic that it is only about theft.  Thieves guilds have always been more like the Godfather type movies or the implied Yakuza/Triad from the kung fu movies.

Quote from Character77006 >>

However while in working with thieves guilds we think of theft and little else, ...

 

D&D showed up after the Godfather Movies.  The whole crime noir movies of the decades before.   Not sure where you got that bit of logic that it is only about theft.  Thieves guilds have always been more like the Godfather type movies or the implied Yakuza/Triad from the kung fu movies.

As I recall the specific inspiration for D&D guilds was the Thieves Guild in Fritz Leiber's Fafhrd and the Grey Mouser stories, which well pre-date the Godfather