The CW's annual Arrowverse crossover has become a massive television event. What started as a pair of episodes that featured characters from Arrow and The Flash interacting has evolved into movie-length stories involving multiple Earths, heroes-turned-villains, and body-swapping comedy. The crux of the crossover is to celebrate the comic world that it emulates, highlighting both stakes and levity when appropriate. These yearly events are always packed to the brim with hidden nods to the comics, television, and films.
"Elseworlds," the 2018 crossover, brings together a lot of previously unseen realms, from Earth-90 to Gotham City. It's going to take a keen eye to catch everything, as there are plenty of Easter eggs to uncover in each episode.
Upon arriving in Gotham City, Oliver reveals he has an old radio journalist contact he knows from before he got stuck on Lian Yu and became the Green Arrow. He says her name in Vesper Fairchild. Fairchild had a brief relationship with Bruce Wayne in the comics.
The second the show cuts to Kent Farm, Remy Zero's "Save Me" starts playing. This was the opening credits theme during Smallville's entire run and another nod to the fan-favorite series.
While having a conversation with Kate Kane, Kara picks a bust of Shakespeare out of a box. In the 1966 Batman series, a Shakespeare bust is what Bruce pulled to head to the Batcave.
When Earth-90 Flash shows up to warn the team about The Monitor's plan he recognized Diggle. He comments that he's not wearing his ring. This is reference to the constant fan theory that Diggle was going to become the Arrowverse's Green Lantern, John Stewart.