It's no secret by now that the producers of The CW's DC-based TV shows--Arrow, The Flash, Legends of Tomorrow, and Supergirl--intend to use a heavy amount of crossover between the shows in their upcoming seasons. With actors like John Barrowman (Malcolm Merlyn) and Katie Cassidy (Laurel Lance/Black Canary/Black Siren) being signed on as regulars across all of the shows, these series' writers have more leeway to use those characters--and more of an obligation to as well.
It has already been revealed that Supergirl will be the focus of the multi-show crossovers, but how that will be done is still a mystery. After all, when The Flash's Barry Allen showed up in an episode of Supergirl last season, it was revealed that the latter show took place in an alternate universe from the one in which The CW's shows are primarily set. Where there's a will, however, there's a way, and having multiple universes collide is a hallmark of DC comics. One way to bring the shows together: by introducing a common enemy.
EW spoke to producer Greg Berlanti about the upcoming season of CW/DC TV shows - and while not everything is in place yet as far as the shows' plots, Berlanti did comment one aspect of those plots that he's pushing for in those plots:
"I’m going to probably try to have one unifying threat, as opposed to different things on different ones, which is what the first year was."
The big question is what that "unifying threat" might be. It has already been revealed that Legends of Tomorrow will introduce both the Justice Society of America and the Legion of Doom, and it is hard to imagine a bigger threat than the Legion of Doom. With Legends hopping back and forth through time, there is also plenty of potential to hop back and forth between universes. That means the Legion of Doom could follow them to Supergirl's universe, or one of Supergirl's enemies could join the Legion of Doom.
The members of the Legion of Doom announced so far--Arrow’s Malcolm Merlyn (John Barrowman) and Damien Darhk (Neal McDonough) as well as The Flash’s Reverse-Flash (Matt Letscher) and Legends of Tomorrow's Leonard Snart a.k.a. Captain Cold (Wentworth Miller)--certainly pose a formidable threat, and represent a cross-section of villains from all of The CW's DC shows as of last season. Newcomer Supergirl is not represented in that lineup, though, and it seems likely that will change. With Lena Luthor being added to that show's roster, might a TV version of Lex Luthor be the final missing piece of the bad guy puzzle? Perhaps, or perhaps the "unifying threat" of next season will be someone or something else entirely.
The Flash season 3 will premiere Tuesday October 4th at 8pm on The CW, Arrow season 5 will premiere in the same timeslot on Wednesday October 5th, Supergirl season 2 on Monday October 10th, and Legends of Tomorrow season 2 on Thursday October 13th.