Just some rough notes:-
The
Bad People Mod (originally by Roger Bacon and recovered and updated by Platyping) has some interesting ideas. For best implementation in C2C we need two things before I would implement it.
1) Crime units that exist just as crime units throughout the game.
2) Most importantly units that are owned by someone but can't be controlled by them! (Note the Immigrant unit should also not be controlable

and is coded that way in the original mod.)
Normal crime units are under the control of the player and can do the same things as those that can't be controlled. Things that can be done are
- one time increase local crime (unit may survive but retreat home) amount depends on gangs etc present
- install "Gang" if tech, crime rate and pop support it. Allows a few auto build crime buildings
- install "Rival Gang" requires "Gang" tech and more pop
- install "Foreign Gang" (only by foreign units) similar to "Rival Gang" but money and espionage go to player who added the "Foreign Gang". If one exists this event replaces the existing one...
- upgrade "Gang" to "Mob" removes all of the gangs in and temporarily increases crime. Allows a bunch of new crime buildings (auto built)
- upgrade "Mob" to "Cartel/Mafia"
Notes:
- these "buildings" may be specialists instead as in the Bad People Mod.
- Gangs produce more crime than Mobs with Cartel producing the least crime. They are all about organisation and have to respect the law. However the extra auto built buildings that come available increase both in number and severity with the increase of the organisation.
- counter: buildings and units have a chance of expelling one of these per turn. There also needs to be a special "great" anti-crime unit that can clean up a town
Just to ping off these thoughts...
This sounds a lot like the Organized Crime I had considered working on at some point waaaay down the road.
The concept I had been considering would be driven by the corporation engine with the criminal organizations acting almost identical to corporations with the exception that they are generally a very bad thing to have in your cities.
Thus you'd have the headquarters established somewhere in your own territory (those would be mostly beneficial to control as is normal for corporations, due to the fact that a lot of money is being generated for the local economy from the criminal operations) and then from there you'd have criminal units that could be built from there that could go, as executives though they'd have more use than just this function as you mention, spread the organization's influence to opponent nations.
That influence would be a wretched, yet hard to detect (requiring espionage most likely) drain on those 'spread-to' cities. As is normal for corporations, each city the criminal organization was spread to would be generating more income for the headquarters.
Thus the whole criminal organization becomes another way to wage a cold war against an enemy, destroying their economy, enhancing their crime levels etc...
Then you'd have ways to counter it and 'clean things up'. Not the least of which would likely be that if a crime level were too low in a city, the organization's influence would be either completely subdued like a deactivated building, or ejected altogether.
In addition, I'd thought this up as a part of the 'Dark Great People' project - it would be something for the polar opposite GP of the Great Merchant (call it Great Scoundrel or something to that extent) - such a GP would be able to found the Criminal Organizations in a similar way to how Merchants are the most common to found corporations. They might be useful much earlier for that than merchants are though, as Criminal Organizations go back a lot further in history than Corporations do.
The Dark Great People mod would have us possess a second GP progress bar that would represent the national total of GP pts being brought in by all cities combined. Thus it would begin to matter what the ancillary cities are generating as far as GP pts are concerned. Obviously, the thresholds would be set for national levels rather than local ones as our normal GP progress bar does. And it would track the normal GP types just as the local ones do except that instead of birthing those types, the type birthed would be the 'Dark GP type' that correlates to the normal one. Thus if you got a Merchant as the birthing result, you'd actually get a Great Scoundrel instead - the unit would come into play in the capital automatically.
I'd also considered we might want to have Cults that operate similar to religions but on a negative basis much like the Criminal Organizations. Obviously, these would be driven by the Great Heretic - the Dark form of the Great Prophet.
I bring this all up because it
sounds like you're making some suggestions and plans that could be very harmonious with these ideas so it might be beneficial for us to consider the 'big picture' of what we might want.