Legends of Tomorrow has already revealed that the Justice Society of America will be a major factor in their second season.
The CW introduced the spinoff series in a two-night crossover event on The Flash and Arrow.
Technically, viewers were introduced to Vandal Savage (Casper Crump), the season 1 villain, before an actual episode of Legends of Tomorrow ever aired.
One week earlier, both The Flash and Arrow premiere on Tuesday October 4th and Wednesday October 5th, respectively, in the same timeslot.