I had some thoughts regarding a fun and realistic way to implement criminal units. I play C2C far less often than the rest of you, so apologize if some of these things are implemented.
1. Spawn criminal units randomly in one of the cities workable tiles. Maybe set this so they spawn in the tile that produces the most crime (or gold). Take into account the terrain, improvements, and culture of the tile when determining what unit to spawn. For example, a hill that was mostly Scottish would likely spawn highlander bandits
2. Indeed it may be a good idea to implement this as a civ, with the same goal of founding it's own cities, etc. But instead of making it one global civ, integrate it with the barbarian civ mod, or use CivRev for inspiration.
2.1 Perhaps lower crime level units would only pillage and such, but as crime level increased the stakes would get higher. For example, at higher crime levels emigration could be simulated by dropping the city size by 1 and spawning a criminal settler and high quality defense unit or two, who then beeline for the nearest and best spot to build a new city.
3. I think someone mentioned this, but units would spawn when the crime level changes and a new crime building is constructed. This would eliminate over spawning and I don't know much about coding, but it's probably easier to code and easier on resources than setting a spawn rate.
4. Implement different AI's for different units, giving them unique goals. Simpler earlier units would simply pillage and hide. Higher level units would become more aggressive, and if conditions were right units would spawn in the manner I mentioned in #2, with the intent to break away.
5. Implement a number of different criminal units based on era, culture group, and crime level. It would allow for more difficulty and awesome flavor. I see 20's USA having a problem with the mob, and 14th century Japan having a problem with Ronin.
5.1 Perhaps when cultures in a city compete, there would be a chance that a unit from either of the culture types would spawn.
6. Vary the size and quality of criminal units as well as the type. Makes for more flavor and less coding? On a side note this could be done for barbarians and animals too. Think a Memory (herd) of elephants vs elephant (group size), pack alpha vs wolf (quality). For criminals think merry band vs. criminal organization (group), ruffian thieves vs. triad syndicate (quality). I don't know much about AI and overhead, but I feel like if animals, barbs and criminals combined into larger groups and used quality promotions, it might actually reduce overhead by lowering the amount of units required to move on their turns.
7. Adjustments to crime fighting promotions? Available to more unit types at earlier tech levels.
8. There are buildings that increase the visibility range of a city, but maybe add the ability to see invisible units within a certain radius for some buildings. Additionally, consider allowing certain defensive improvements (like the watch tower) to be built on top of other tile improvements. Their visibility range could be reduced, but they could help to reduce crime in the tile.
9. Great Criminals and Great Lawmen (Al Capone / Wyatt Earp)
10. Implement a system that allows civs to use gold to pay off these criminal units and then employ them similarly to privateers. Make them employable for a certain amount of turns, after which they go back up for hire at a steeper price. This would introduce a mercenary element to the game (something prevalent in history but missing from every civ).
11. Replace the pillage system. Make it more like the espionage system. Make new missions for criminal units and other harassers like the rogue. These missions would allow units to syphon off other yields from the tile they're on. They could hijack a train to steal production, raid estates to steal culture, commit highway robbery to steal wealth or interdict trade, observe movements to increase espionage or raid supply convoys to deny a resource. They could steal these resources and send them to their nearest city, gaining experience and working towards a great person.
12. Make new types of law enforcement units and increase their prevalence outside the city walls. They would have counter-missions to the ones I mentioned above, as well as a few unique ones. They could protect a trade route (increase trade income), guard a mining operation (increase wealth or production), run checkpoints (counterespionage), or interdict immigration (cultural influence). In a city these same units could fight specific types of crime (violent crime, drugs, maybe even truancy). In addition to reducing the overall crime level, each mission would have a unique positive effect. For example fighting violent crime would increase happiness, fighting drug crime would reduce disease or increase health, enforcing truancy laws would increase education, and investigating white collar crime would increase culture.
11 and 12 would be incredibly cool, allowing the player a greater deal of customization over their cities and important tiles. It also creates a sort of cold war effect, allowing two civs to exchange blows for years without ever being at war. And of course, the same system would also allow the implementation of a fun and unique crime system. Let me know what you guys think.