My preference for World Out-Look/Power Sources:
The Wake of the Comet, in the 1990s, the coming of Haley's comet brought about some rather astounding events. Whether it was the comet, which passed closer to earth then ever before, or if its just a coincidence, people started expressing superhuman abilities. The phenomenon was quickly labeled UGE (Unexplained Genetic Expression), because for some reason, scientific sounding acronyms reassure people, even if they are totally meaningless. From the very start, there were those with powers that prefered to benifit from them at other people's expense, and that group has grown as more and more people develop powers.
Just as there are those that use their powers to take what they want, there are others who use their powers selflessly for the good of others. In the 1990s these heros worked ainly solo, though there were exceptions, but since then, there have been a number of situations that have arisen to force the heros to band together to protect the populace from super-powered criminals, and even mother nature. These crisis became more and more frequent as the number of villians have increased over time. After a particularly violent and deadly episode, some of the most famous heros created a permanent team to deal with the rising menace. That was almost 5 years ago, since then, they have expanded their roster.
My preference for the framework of super heros in the city would be:
JLU (or whatever name)
Founders/Senior Members:
The founders created the league, they are the examples that heros try to live up to, they make the major decisions for the league, they give out the assignments to the members. When working with other members of the league, they are mainly in the mentoring role as their power level would place them above the rank and file. NPCs for Storytellers to use to bring the character's into the plot, and for use in meta-plot changing events.
Members:
These are the mid-level heros, they are competent, with a bit of experience, they are the ones that go out and get things down. These would be the PCs, they are at the power level of the campaign world. They are assigned the missions, they go out and beat the bad guys, they are the ones down in the trenches so to speak. They have some say in the major decisions of the team, in that all members can give input on the major decisions, but they are left out of the day to day decision making for the most part.
Provisional Members:
These are the heros just coming into their powers, the ones who aren't sure of themselves yet, or simply don't have enough experience to be counted among the other members, after a period of working within the team, they are promoted to full members, and join the ranks of those depended on to keep the city/country/world/whatever safe. They'd start off at a lower PL, but with PPs being rewarded for play, they would make it up to the campaign PL eventually, at which point they'd be full members. Think Robin as he was working himself up to Nightwing, when he became Nightwing he'd be a full member.