There is seriously something magical about super heroes. They're larger than life, and the things they do have caught the eyes and imaginations of gamers and geeks the world over. The idea behind this game is to try and cash in on that feeling, and bring it to the table.
The game takes place in a world much like our own one, social media has exploded all over the world and things are going about as you'd expect, with one exception. That exception is this: there are super heroes in the world. Now, when the Super Heroes first showed is something that the players can go over with the GM at the time, but the idea here is that maybe 5-10 years of super heroics has happened. A few have established themselves as the big boys of the universe (the Supermen, Batman, Ironmen, etc.) The point here, we've got all the modern technology, and an explosion of heroes and villains with super powers.
The fun part of this game, at least in my opinion, is the fact that this game can house a very large number of players. The idea here is for it to be a game where people can drop in or out as they want, and can run on little to no notice. Tim and Susie can't make game on Friday? That's fine, their characters just aren't there that week.
Now, there would be some special privilege for those who joined in earlier. Namely, they'd get to define (or play) the Top Dogs when going over the universe. Not that others can't be just as powerful (Captain Marvel and Superman are equal in terms of power, but Superman is one of the Big 7, Captain Marvel is not), but they won't be afounding member of the League. Players will also be allowed to have multiple characters of different power levels and types if they so choose. The idea here is for lots of heroes to be present after all.
There is no specific where for this. Players will be sent to do heroics all over the world, wherever they are needed. When game starts, whatever characters are there get sent to a mission where they get to be heroes. Each session will be episodic in nature, with over-arching plot threads strung carefully together through multiple episodes to get the story across. Also, considering the range of power levels that will likely be in the game, choosing to have it be a global game just means not having to try to keep people who can fly at Mach 6+ in one location when they want to go elsewhere.
Most of the what should already be apparent by now. The PCs play the various members of the League of Heroes. This is an entity somewhat like the Justice League from Justice League Unlimited. In response to the rise of super villainy and regular threats to the world, the heroes of the world have loosely allied and are working to organize themselves to do a better job at saving lives.
Through this, Players would have to deal with a number of complications. Primarily, the threat from the various villains that they are going to be facing off against. Secondarily, the fact of being a super celebrity thanks to their hero status, along with all that comes with it in this world of social media. Tertiary, the consequences to their actions based on that fame, and how they handle the problems of villainy.
The game is supposed to feel Heroic, and yes, that is a capital H for a reason. This isn't supposed to be an iron age game, where the heroes are fine with killing civilians if it gets the job done. The idea is to be non-lethal more often than not, and with consequences for those who keep going lethal. This is part of the point of the feel though, I suppose. I won't stop anyone from going Iron Age out of character, but they will have to deal with the consequences of those actions. In general, the concept of consequences and the ever famous line about power and responsibility will be the key guides to feel here.
System wise, I think Mutants and Masterminds is a good choice. There are other games out there that could work, but I like the combination of simplicity and flexibility M&M has. It is also something that most of the people around here already know, and can be easily explained as its roots (however far 3rd ed is from that) come from D20 and D&D.