Five years ago, the people of earth thought they were doomed when the sky started to burn and boom tubes opened all over the globe. Darkseid's parademons struck at every corner of the earth, creating fire pits and abducting humans to be used as genetic material for the great war engine. When hope seemed lost, heroes banded together to maintain order. Superman, Batman, Wonder Woman, Hal Jordan, and a host of others attacked the boom tubes, forced the parademons back, and even confronted Darkseid himself. Their actions inspired the earth and gave it a second wind. Unfortunately, most of them gave their lives in the final battle.
In the present day, earth is still recovering from the damage Darkseid did. More importantly, they are learning to cope with their losses. Five years has given the criminal element plenty of time to adapt to the way of things. They know there is no longer a caped crusader or man of steel to stop them, and they have become far more bold. Worse, everyone knows that Darkseid isn't truly gone. He will rise again, and when he does, this time he will find earth lacking the most vital part of its defense.
The time has come for new heroes to rise.
This is an idea lifted from DC's current Earth 2 run. If you aren't familiar with the book, it covers a new group of superheroes struggling to protect a world that has lost its champions. Many of the heroes pay homage to fallen idols by taking up their mantles, but they are not perfect copies of the originals. For example, in the book, the new Superman is a second kryptonian survivor, but one who has sworn himself to pacifism. He protects the earth, but will not use violence. Some heroes are completely new. In the book, Lois Lane's mind has been uploaded into an android body, and she is now Red Tornado. I don't want to copy the Earth 2 book too much, so I'm just using these as examples. Some characters might be grown up sidekicks like Robin or Kid Flash who are ready to take on the mantle of their mentors. Others might be completely new characters who are ready to take up existing titles, or perhaps create their own.
If there is enough interest for this to work, I would probably play as a new Dr. Fate and use him as a catalyst to bring the group together.