I've unsubbed from the old version, but hows this:
Thirty years ago Darkseid attacked Earth. Almost every Hero and Villain tried to fight them, but Earth lost and most of those that fought back died. Some of the survivors managed to escape with a few hundred thousand survivors, including those that were evacuated by the few aliens that answered Earth's distress signals, before Darkseid glassed the planet to guarantee that it never again stood against him. Star Labs, Wayne Enterprises, Lexcorp, and several others managed to get ships off of the planet along with heroes, villains, or other superhumans. Most of those that escaped became Darkseid's subjects, but some found refuge on other worlds.
A few weeks ago Lex Luthor, now one of Darkseid's best scientists, secretly contacted several of the descendants of these refugees and asked them to meet him at a specific location on Earth.
There he told them his plan. He had built a time machine where Metropolis's Central Park used to be, as it sent you back to the same geographical location, and planned to send them back to just before Darkseid's invasion with information the heroes and villains could use to fight back and hopefully defeat it. He chose the descendants because he needed people he could trust, but who didn't already exist in that time period, hoping to minimize the butterfly effect.
Luthor has a backup plan, but hopefully he won't have to use it.