Arrow season 8, episode 3, "Leap Of Faith," ended with the final destruction of the League of Assassins and, with it, the formation of a new group that could become the Justice League of Earth-1. Such an occurrence could not have come at a better time, given the approaching crisis that threatens the whole of the Arrowverse.
The action of "Leap of Faith" centered upon a three-way race to the tomb of the League of Assassins' founder, Al-Fatih. Two splinter-factions of the League of Assassins, the Thanatos Guild and the followers of Talia Al Ghul, sought the tomb to claim Al-Fatih's sword, which was seen as a symbol of rulership among the assassins. Oliver Queen and his sister, Thea Queen, sought the tomb as well, though Thea aimed to keep the sword from being used by any faction to revive the League of Assassins, while Oliver was looking for the journals of Al-Faith, hoping to find ancient knowledge regarding the cosmic being he knew as the Monitor.
In the end, the Thanatos Guild was destroyed and Talia bested in single combat by Thea. Talia agreed to surrender command of her students to Thea, proclaiming that it was clearly Thea's destiny to rule the League of Assassins. Thea had other ideas however, saying that she didn't believe in fate or that the world needed a new League of Assassins. What the world needed instead, Thea declared, was "a League of Heroes." Thea offered Talia a place co-leading this new League and the eldest daughter of Ra's Al Ghul accepted.
This reference to a "league of heroes" is only the latest in a number of teases the Arrowverse has made regarding the formation of the Justice League. The first came during the Invasion crossover event, where the heroes of Earth-1 and Supergirl operated out of a hangar owned by STAR Labs that bore a strong resemblance to the Hall of Justice from the Superfriends cartoons. Another hint was dropped in The Flash, season 6, episode 3, "The Death Of Vibe," when Nora West-Allen made reference to all the superheroes who tried and failed to capture Cicada, noting that "Supergirl, the Legends, even the League tried, but no one was successful."
While this may seem like an improbable origin for the Justice League, it is no more unlikely than the Laurel Lance of Earth-2 (who served under four different criminal masterminds over four seasons of Arrow) seeking redemption for her crimes as the new Black Canary. As Thea noted when she offered Talia a place by her side, "we've all made mistakes." Besides, even if this new League of Heroes does not transform into the Justice League, the Arrowverse needs all the heroes it can get.