Conspiracy buffs are fond of pointing out unexplained events or odd coincidences and blaming them on the Illuminati, those behind the scenes rulers of all. They're a great hook for a campaign. No matter how much you think you've uncovered you always have more plots, more reveals,and more conspirators.
I'm inverting that. No one can keep all their secrets all the time with no down time. The upper echelon of any organization need someplace, a retreat if you will, to unwind. Thus the Illuminati came together long ago (or recently, the records are ... incomplete) and founded Sanctum.
Sanctum is the one place on Earth the Illuminati do not rule (though each group does influence various sectors.) The city government enforces a strict neutrality among all the conspiracies. All f the conspirators come to Sanctum to relax and unwind and even talk shop. Some live as citizens of Sanctum their whole lives. Others join one of the Secret Masters. It's easier for some than others. Each of the cabals has a duty they perform in the city, all supervised closely by the city fathers. Some highlights:
The Bavarian Illuminati are subcontracted to handle many services: water, sanitation, power, and maintenance. The water is distilled, not treated with the common chemicals that arrest people's psionics and illumination. Sanitation trucks all have onboard incinerators (houses have incinerators too but you can't be too careful with your trash.)
The Internet, and media services are maintained by the Network. The consensus is they monitor people about as well as the NSA or FBI but the service is way more reliable. Conspiracy theorists would sell their souls to watch an hour of the reality show channel. There are also no viruses, spamming, pop ups or other distractions.
Sanctuary's banks and monetary functions are handled by the Gnomes of Zurich who provide very high interest savings accounts and many free services. Most people use debit cards or gold to pay for purchases to avoid dealing with minting currency etc. The Network and the Gnomes both watch over the electronic payment system to make sure no one's privacy is violated (at least in theory.)
Many citizens of Sanctuary wish to become part of the conspiracies and see the outside world. Citizens are not allowed to leave unless they join one or another conspiracy. Citizens with a scientific background try to impress the Masters with some kind of technological breakthrough often with mixed results. It's difficult to get an audition with the Masters though and some try to get their attention through committing crimes or juvenile pranking. The city government takes a tough stance with them. After all, if Sanctum locks up one of these nut cases before they can be recruited by the Masters they have a bargaining chip.
Some scientists working for a conspiracy are given intelligence enhancing drugs. These are not allowed by the city government. Scientists given the drugs become dependent on them and risk mental problems if they suffer withdrawal. A few do risk it and try to hide out in the city. These are hunted down by the municipal police as well. They can be traded back to their bosses for a sizable reward in many cases as well.
Besides the Illuminati, the city is also a retirement community for prized conspiracy agents. Lesser agents are mind wiped or ... dealt with. The city also is home to many experimental subjects of the Masters' labs who didn't turn out quite as planned. Some of them join the Municipal Police Force, since they are very unlikely to be agents of the Masters.
Some groups do not play well and are not in Sanctum in large numbers. A few Discordians are entertainers and run a small embassy for recruiting. The Fourth Reich disdains the creature comforts and decadence of the city and has its own recreation areas for its minions. Higher ups do get in sanctum of course but under the guise of staffing their recruiting office.
The city government is relatively free of Illuminati agents. It has to be as it enforces the neutrality of Sanctum and forces the various factions to play nicely. For the most part the Illuminati play fairly here. The treaty all who participated in is very simple: no acts of violence, no infiltration or influence of the city government, abide by all local laws. breaking the rules can result in loss of privileges, jailing or fines.
To enforce its laws the city uses a corp of Men In Black as well as various special agents augmented by cybernetics, drugs, psionics, and alchemy. This is only prudent, we are talking about the Illuminati.
Where is Sanctum? there are a lot of theories on that by the citizens. A force field/minefield/dome covers the city. No one is quite sure what the boundary is. Some who approach it are never seen again. Others return with their minds wiped. The scariest theory is that the missing explorers are mind wiped and sent to live in the outside world, boring place that that is. Some say it is in a pocket dimension. Others believe it is accessed by time travel and is in the distant past or the unknown future. Some say it is on another planet entirely. The agents and Masters who come from the outer world are mind wiped. It is a mystery many would like to penetrate.
The characters are Men In Black tasked with tracking down scientists who have gone off the grid.
The characters are retired agents looking for a big score to provide for their golden years. There is still 'normal' crime in the city.
The group works for Animal Control. Of course in Sanctum that can cover anything from the Moth Man to intelligent insects.
The characters are blue collar citizens tasked with construction. They receive all kinds of offers to plant bugs in the building they are currently working on. Do they got to the Men In Black? Do they take the offers? What about the other guys they saw planting bugs as they were leaving their shift.
A noted Illuminati scientist hires the group to procure several ingredients for his next experiment from the depths of the Undercity.
The group is hired to recruit for a group. They get to answer the questions of jaded citizens, annoying teens, and deal with the occasional double agent.
Everyone wakes up near the boundary mind wiped. Only they did the job a little too well and aside from their names the characters remember nothing. Create your characters as you go filling in skills/cliches/whatever.
A major local corporation seeks to fund a Super Hero Team for PR. You can either be the corporate spin doctors or the supers with no clue who is in more danger.