One of my ideas was a superhero game. You had a typical slightly futuristic city with a variety of district types including a large area of slums that didn't get the same degree of protection as the better parts of the city and hence were an area which a lot of criminal gangs used to hide out. The city had an organised team of superheroes, with senior and junior sections, as well as some powered vigilantes operating in the slums. Then there were supervillains from the good parts of the city who regularly fought the super-team in disguise and carried on their normal lives - people knew but couldn't prove that a well known businessman and philanthropist was also one of the nastier powered crime bosses.
And then the super-team lost, the bad guys took over, and the players are remnants of the junior members of the team and various others surviving in the slums trying to organise a resistance that would take back the city from the crime lord(s) who now ruled it. Having to make deals with the gangs, survive the attempts to track them down by lesser supervillains, even just getting by day to day in a district where their previous wealth and status meant very little.
Some of the influences: the Dark Angel TV series, the Heroes Rise text adventures, the Avengers (Marvel) comics, Shadowrun, and various gangster movies.
I still might run this one day, if I can get a rules set that I like and my potential players would tolerate.