In the closing seconds of "Elseworlds, Part 3," The CW revealed it will adapt the seminal DC event series Crisis On Infinite Earths for the fall of 2019 with a teaser featuring the logo for what may likely be the next Arrowverse crossover.
Stephen Amell shared the image to his Twitter account shortly after "Elseworlds" ended, with his only caption a wide-eyed emoji.
In the seminal comic book event series Crisis On Infinite Earths, the multiverse came under attack by the Anti-Monitor, who sought to destroy all life. To combat this threat, the Monitor assembled the greatest heroes and villains from each world to fight back. By the time the Anti-Monitor reached Earth-1, only five worlds remained. Under the Monitor's guidance, the metahumans of all five Earths banned together and overcame the Anti-Monitor -- but not without some causalities. Crisis On Infinite Earths is also remembered for the deaths of Barry Allen and Kara Zor-El, as was teased in the final episode of "Elseworlds."
Notably, Crisis on Infinite Earths merged the remaining Earths together. If The CW's "Crisis On Infinite Earths" follows the source material closely, that could mean Supergirl could ultimately join the Arrowverse proper with Arrow, The Flash and DC's Legends of Tomorrow.
It is unknown at this time if "Crisis On Infinite Earths" will be an official crossover encapsulating the different Arrowverse series, or a special event taking the place of the aforementioned Arrow, The Flash, Supergirl and DC's Legends of Tomorrow.