From the past to the future, seldom-seen super-teams made appearances in Doomsday Clock #5, revealing that the event may culminate in a much broader DC Universe.
A raging fire at a steel factory glowed green, catching the eye of Johnny Thunder. He escaped from the institution where he lives and took a bus out of town, looking for an item he was sure would be there: Alan Scott's power battery/green lantern.
Once there, he was approached by a gang of youths who were ready to assault him. It looked momentarily like he might suffer the same fate as Hollis Mason, the original Nite-Owl, who was murdered by a similar gang in Watchmen...but he was saved at the last moment by Saturn Girl.
And, yes, at this point she is calling herself Saturn Girl and wearing the costume. Paired with Rorschach, she sought out Thunder, and the unconventional trio are ready, as of the end of the issue, to tackle new challenges.
The Justice Society of America were DC's earliest all-star super-team, and were later retroactively moved to Earth-2 when the Silver Age reinvented many of the members and eventually introduced the Justice League. After Crisis on Infinite Earths, all of DC's realities were merged and the JSA were reinvented as the "original heroes," relegated mostly to the distant past, before they were revived by Geoff Johns and David S. Goyer. They have not appeared in their classic form since the 2011 New 52 reboot, but a version of the team, updated for the current day, appeared in the Earth-2 comics released for a few years after that reboot.
The Legion of Super-Heroes, teen superheroes from the future influenced by Superman's reputation, have been constantly reinvented and retooled since Crisis, when the altered timeline removed Superboy's physical presence from their lineup. The team did exist post-Flashpoint, unlike the JSA, but have had a pretty minimal role since the end of 2012. Their most recent appearances, outside of several cameos by Saturn Girl in various books since Rebirth, took place in the now-defunct Justice League United title.
Bear in mind that while the events of Doomsday Clock are canonically taking place in the DC Universe, they are taking place a year from the current stories, giving the DCU a chance to catch up. So don't expect a Legion of Super-Heroes or Justice Society of America comic out in a couple of weeks just because of these little moments.
Doomsday Clock #5 is on sale now. The sixth issue will be available in stores and online on July 18.