Marvel Universe: Next Avengers-1
"And there came the day, unlike any other. When Earth's mightiest heroes were united against a common threat. The Soldier, the God, the Knight, the Spy, the Giant, the King, the Pixie, the Ghost, and the Archer. On that day, the Avengers were born. To fight foes that no single hero could withstand: time-traveling conquerors, alien invaders and masters of evil. The Avengers vanquished them all. And finally, when the world was at peace, the heroes built lives of their own. The Soldier and Spy fell in love, as did the Giant and Pixie. The King found his Queen. And the Thunder God returned to his kingdom far away from the world of man. And one by one, the children of the Avengers came to be. Children who would one day become a new generation of heroes. But in time, evil returned. An evil called Ultron. Ultron wanted nothing less than total dominion over the world. But the Avengers stood in its way. Sadly, their defeat was inevitable. But before they fell, the Avengers hid their children in a faraway place, where they would grow up safe from Ultron's grasp. Because the Avengers knew that as long as their children were alive, there was hope. Hope for the future."
Earth-555326. A post-apocalyptic future in the early 21st century.
About fifteen years ago, after a period of peace during which the Avengers were able to relax, establish families and have children, Stark's creation -- Ultron -- rose up and defeated the Avengers, killing many of them. Even as the Earth's Mightiest Heroes were falling Tony Stark took most of the next generation into hiding.
With more than a decade effectively unopposed, Ultron's influence spread across the globe. Heroes and villains who opposed him continued to fall only to be featured on a trophy wall in Ultron's city-sized citadel located where New York City used to stand. When accidentally alerted to the location of his creator and the Avengers' children Ultron tried to end their threat only to be defeated by them, with some additional allies, and then thrown into space before he could reconstruct himself.
Now the world is no longer under the thumb of its conqueror but the remains of its robotic forces are still active in pockets and the recovering human populace need heroes to help them rebuild and to rebuff these lesser but still capable AIs' attempts to subjugate them again.