The Justice League has just become an established part of The Flash's future timeline - but it seems they failed to capture Cicada. Blink-and-you'll-miss-it dialogue from Nora in the latest episode, "The Death of Vibe", saw her refer to the Justice League.
There's been a lot of speculation that the Arrowverse heroes will ultimately form an alliance, creating the Justice League. After all, counting the Supergirl reality, a lot of the major heroes are in play - Superman, Green Arrow, the Martian Manhunter and the Flash are all major Justice League members, while characters like the Atom, the Elongated Man, Black Canary, and even Supergirl and Black Lightning have been part of the roster on occasion. With so many heroes in play, surely the Justice League is inevitable?
The latest episode of The Flash, "The Death of Vibe," sees the first explicit nod to the idea we could get the Justice League in the Arrowverse sooner or later. In one key scene, Nora reels off a list of heroes who attempted to hunt down Cicada in her timeline; she names Supergirl, the Legion, and - significantly - "the League." Presumably, in her timeline Supergirl traveled over from her own dimension to hunt Cicada at some point, while there could easily be a Legion of Super-Heroes in the Flash's reality as well (we've seen Legion flight rings on other Earths, suggesting the Legion isn't unique to Supergirl's dimension).
The Arrowverse shows are now pretty much guaranteed to feature an annual team-up story; this year's Elseworlds crossover promises to be the boldest yet, uniting quite a lot of heroes who've traditionally been part of the Justice League. It's already been confirmed to involve Supergirl, the Flash, Green Arrow, and Superman; if the shows bring along their supporting cast, we'll also have Martian Manhunter, Elongated Man, and Black Canary as well. The event will also introduce the Arrowverse's version of Batwoman, who could easily serve as a substitute for Batman. It would hardly be a surprise for Elseworlds to introduce the idea of the Justice League.
Warner Bros. has typically been wary of risking diluting their most powerful brands by exploiting them on both the small screen and in the movies; that's why the Suicide Squad and Harley Quinn were dropped from Arrow. There are rumors that strategy may well have changed as a result of the AT&T/Time Warner merger, though; in any case, given the poor box office performance of the Justice League movie, it wouldn't be a surprise to see the DCEU drop the team-up idea entirely. All this means the Arrowverse may finally be free to unite its own League.
One thing is important to remember, though; Nora has changed the timeline, to the extent that Cicada has become an entirely different person. That means everything is in flux; the Justice League may never form in the first place. If they do, though, there's no reason they have to fail to bring in Cicada as part of this timeline.
The Flash season 5 continues with episode 4, "News Flash", on Tuesday, October 30th at 8 PM on the CW.