The ancient Roman and Byzantine empires had well-developed associations, known as demes, which supported the different factions (or teams) under which competitors in certain sporting events took part; this was particularly true of chariot racing. There were initially four major factional teams of chariot racing, differentiated by the colour of the uniform in which they competed; the colours were also worn by their supporters. These were the Blues, the Greens, the Reds, and the Whites, although by the Byzantine era the only teams with any influence were the Blues and Greens. Emperor Justinian I was a supporter of the Blues.
The team associations had become a focus for various social and political issues for which the general Byzantine population lacked other forms of outlet. They combined aspects of street gangs and political parties, taking positions on current issues, notably theological problems or claimants to the throne. They frequently tried to affect the policy of the emperors by shouting political demands between races. The imperial forces and guards in the city could not keep order without the cooperation of the circus factions which were in turn backed by the aristocratic families of the city; these included some families who believed they had a more rightful claim to the throne than Justinian.
The first thing I thought after reading this was making a RPG or board game about the gangs of Constantinople. But after a few moments I realized there is nothing really novel about it; most RPGs I've played were already about gangsters.
And I mostly play standard D&D stuff.
When I wrote about what makes a good setting I listed shades of grey, multiple interesting factions and a stable (but malleable) social order, among other things. Add "violence" to the mix since this is one of the "pillars" of most RPGs.
I mentioned Game of Thrones as a good example. Since there are no new books, I will say I still watch the show. I haven't found other good fantasy shows before or since, but there are plenty of other series I enjoyed. The Sopranos. The Wire. Boardwalk Empire. The Shield. Sons of Anarchy. Orange is the new black. Do you see a pattern here?
I also watched Vikings for a while. I eventually got bored, but I watched enough to realize the Vikings are exactly like the Sons of Anarchy (a 1%er biker gang) with better plot armor.
It doesn't matter if they are illiterate, violent or lazy; they are the heroes, because there are worse people out there, and at least they are brave and protect their own.
In short, they are the player characters.
Well, not necessarily. Maybe your PCs are more like Eddard Stark - but Ned still allied with a gangster pimp to try and make things right! And all those Lannisters, Boltons, etc., are obviously mafiosi. Their so-called "honor" in not different that following a code such as the Omertà. Or "don't ever take sides with anyone against the Family".
Which is why stuff like this feels so fitting:
|Ricean Vlad - Source.|
Anyway, this is not quite the "eureka!" moment, because it is really obvious. But once you realize why Sons of Anarchy is called "Hamlet on Wheels", and that The Warriors is an adaptation of Xenophon's Anabasis, you have an useful trove of inspirations for your adventures.
The Crystal Shard, for example, didn't add much to my games, but Gangs of New York and The Borgias did. The Wire is a bigger influence for me than The Dragonlance trilogy. Gomorrah (the movie, not the book, about the Neapolitan mafia) might give you a decent insight on how the "big players" of the setting look at novice adventurers.
I know this is not for everyone. You can have great adventures without those things. For example, you might have a dungeon-crawl with puzzles and monsters, even intelligent one, but not factions. You can focus on exploration instead of politics. You can focus on the external threat instead of a higher authority or the interactions between different game. Maybe your antagonists are just evil, and the PCs ally with the good guys to defeat them - including the king, who is honest and loved by all.
But for me, personally, adventures with violent or scheming factions competing against one another - and, specially, the PCs - are some of the best I've ever had.