Ancient Rome is one of my perennial favorites in terms of settings. I'm not entirely certain why I find it so fascinating, except perhaps because we seem to know so much about it. There are so many similarities between we postmodern Americans and the ancient Romans, and yet at the same time, there are differences that really make them seem quite alien at times.
Take, for example, the Roman virtues, which are nicely illustrated in the RPG FVLMINATA: Armed With Lightning. As an example of Duty and Respect, the book proposes a situation based on historical precedent: You find yourself in a civil war. Both sides suffer heavy losses. Your sister's fiancee fought for the other side and was slain. Your sister does not show up for your victory celebration, choosing instead to attend the funeral of her betrothed. As a good Roman, what do you do? Run her through with your gladius for being unpatriotic. One must remember that Christian virtues such as Love, Hope, and Forgiveness were unusual, and often ran counter to typical Roman thinking.
Anyway, I've accumulated a number of books on ancient Rome and find myself coming back to it constantly. I have a couple of maps of Rome (and one of Ostia, Rome's primary port), lots of illustrations, and at least three RPGs devoted to the setting. For quite some time, I've been wanting to convert the Green Ronin Freeport adventure trilogy to a Roman setting. I've also worked to some degree on a comic project in which the protagonist is a vigile, a night watchman. I often describe the project as "Law and Order" set in Ancient Rome. My research on that led me to do some general research on the topic of crime, the forms it takes, the kinds of organizations that profit from it, and so on.
Then an idea was sparked after I participated in a discussion thread on RPGnet. While talking about the notion of "sandbox settings", in which there is no metaplot to speak of, but the setting is created to enable Player Characters as much freedom over their activities as possible. There is sufficient detail in these settings (or the means are present to create sufficient detail off the cuff, on demand) that the Players can choose what they want their characters to do, where they want them to go, and basically follow their own personal goals without the GM trying to railroad them to following a preconceived plot. A good example of this is the "Grand Theft Auto" series of video games. There is an overarching plot, but players are not bound to follow it. Instead, you can spend as much time as you want driving through the city, committing various crimes (because, let's face it, if a game allows you to crash into things, drive like a maniac, and beat people senseless, most of us will do it at least once) as you like. MMORPGs sometimes have an element of this, too. But no video game (thus far, anyway) can really capture the freedom a Player can enjoy in a tabletop RPG designed with this purpose.
Cutting to the chase, someone starting talking about "Grand Theft Auto" in various settings, and someone mentioned Ancient Rome (referring to the idea with the groan-inducing term 'Tiberpunk'). That started my gears turning. FVLMINATA does a nice job of breaking down Rome into its various neighborhoods, giving a fair description of the feel and typical content of each neighborhood in the city. This made it easier to figure out what kinds of crimes might occur where, and where ethnic and economic tensions might make things more interesting.
I thought of the various ethnic groups in Rome which would likely band together for protection, possibly forming gangs to that end. There might be something akin to terrorists, fighting to bring down the Roman government and re-establish their freedom (like the Egyptians or the Celts, perhaps). There would certainly be a black market of some sort, and cartels dealing in proscribed goods. Senators would likely have agents working on the streets toward various ends (especially since Senators were forbidden by law to make profits from business, which I'm certain they would try to skirt around), adding potential extra danger to interfering with any given criminal activity - you never know if you're disrupting a powerful Senator's income! There are the sports teams and their fans, which were such a potent political force in later Byzantium that they caused such riots as to threaten the Emperor's rule. You've also got the regular run of muggers and other petty crooks - Nero himself was supposed to have snuck out to roam the city at night, mugging people... And, of course, there would likely be vigilante groups dealing with all of the above chaos, trying to restore order to the viae in their own way.
Thus, the basis for CAPVT MVNDI. Caput Mundi was a nickname for the city of Rome, meaning "the head of the entire world." This game would be a sandbox setting, in which the PCs are free to roam and explore as they will. Player Characters could work for law enforcement, being police/detectives or lawyers, rooting out and fighting crime while dealing with a sometimes corrupt government; or they could be up-and-coming criminals trying to make a place for themselves in a wicked old city.
On a tangentially related note, I watched the pilot episode of the short-lived television show "Roar" on YouTube yesterday. Notable now, perhaps, for starring Heath Ledger as the show's protagonist, it was set in a Celtic Britain that is held (or being invaded by, as the episode's conclusion suggests) the Romans. I don't know exactly when it's supposed to take place, but given that Longinus shows up and mentions that he's 400 years old, I'm guessing it must be sometime around 365 AD (assuming he was about 35 when Jesus was crucified). This would mean that the Roman hold on Britain would only last about another 45 years before the Romans there declare their independence and are told by the Emperor to "look to their own safety." I just looked up some data on Rome at that time and it seems to jive with one or two other things I noticed in the show (for example, one of the Roman noble characters present seems to insist on a monotheistic faith; 365 would be post-Constantine). The show has a little bit of cheese - things that probably looked good on paper but came across as a little hokey in live action - and the Hero's Journey structure was pretty apparent to me, but it wasn't bad. The pilot episode is pretty self-contained, like a miniature movie, so while I think there could have been more open ends to entice the viewer to continue watching the series, I am kind of interested in seeing where next they went with it.
Watching the episode made me think, not for the first time, that a game in which players play Celts fighting Roman domination of their lands could make for an interesting campaign.