Game System: Initially, I had thought this would be a perfect fit for Steve Kenson's wonderful Icons, but the more I think about it... It can only be Mutants and Masterminds for this one.
Nutshell: A Power Level 8 or 9 Iron Age Style Superhero Campaign set in a 4-color world (Freedom City, DC, Marvel, heck even Superhuman's setting would be ideal). It is initially meant to be a street level campaign but grow from there.
Concept: The Crew revolves around the PC's, a group of super-criminals brought together by a mysterious benefactor. Each one has super powers, but not of the Superman/ Green Lantern level of power. But most importantly, each PC is a criminal. They don't have to be inherently evil, in fact the game works better if they are not. They are simply criminals, worried more about their next heist than killing or taking over the world. Also, since they are a team, the PC's powers should not overlap too much as each member would be something of a specialist in The Crew.
To avoid the whole "getting to know you" period, it is assumed that the initial starting adventure would not be The Crew's first heist together. It is also assumed that they have all done jobs for this "mysterious benefactor" before to keep questions to a minimum... at least initially.
The Introductory adventure sets the scene with a bank heist. Unlike most games of this genre, the PC's are the criminal's perpetuating the heist. They would be allowed to embellish and plot and plan and be as cruel or as mercenary as necessary. Of course, it wouldn't be long before the police are called in and a simple bank heist becomes a hostage situation. But that's when things change.
An hour or so into the first adventure, the cop cars will pull away from the bank, called to a larger menace. Anyone looking out the high windows of the bank would see why. Someting terrible has come. Depending on the world, it could be an invasion from Omega, Braniac, the Skrulls, the Monkey Menace, zombies, or some other alien race bent on the world's destruction; it could be a giant robot or creature; it could be a war of the gods. It doesn't matter, it's bad. In fact, it's so bad, every superhero the PC's have ever heard of are involved in the battle that erupts. And they're losing. For all that it begins to shake a bit at the seams and thnkgs like falling statues and stuff might happen, it should seem to the PC's that inside the bank is the only safe place.
So now what do they do? The PC's aren't terribly evil. So do they protect the people in the bank? Do they hunker down and wait for the danger to pass? Or do they help the heroes? If they don't, the heroes surely all die or are seriously injured in the battle. This potentially makes these criminals some of the Earth's only defenders. What then?
Those are the questions I like to ask and that's what I think would be fun to explore.