Brandon Routh's Superman being a part of the Arrowverse was foreshadowed three years before it was officially announced he would return to the role as part of the Crisis on Infinite Earths event. While the realities of other DC Comics' television series have been confirmed to be a part of the Arrowverse, this is the first time the Arrowverse has been suggested to encompass movie universes as well.
Routh was a virtual unknown when he was cast to play Clark Kent and Superman in 2006's Superman Returns. While the movie was not deemed successful enough to merit a sequel, few found fault with Routh's portrayal of the Man of Steel. Years later, Routh was cast to play the role of the genius inventor turned superhero Ted Kord (aka Blue Beetle) in season 3 of Arrow but rights issues with the character saw him changed to Ray Palmer (aka The Atom). Routh went on to become one of the stars of the spin-off series Legends of Tomorrow, where he remains a key component of the Arrowverse's success.
While Arrowverse fans were shocked by the reveal that Routh would play Superman in the upcoming Crisis On Infinite Earths event, this inevitability was cleverly teased three years ago during Invasion! - the first major crossover event between more than two Arrowverse shows at a time. The tease came at the end of the final episode of the crossover, as the heroes of two Earths celebrated their victory over an alien race called the Dominators.
As the party is winding down, Barry Allen (The Flash), Oliver Queen (Green Arrow) and Kara Danvers (Supergirl) walk off to have a talk alone, moving past Ray Palmer and Felicity Smoak. Seeing Supergirl in her civilian guise, wearing glasses with her hair tied back in a ponytail, Felicity (who sports a similar look most of the time) dryly comments that looking at Kara is like looking in a mirror. Ray agrees there is a resemblance, adding "You know what's funny? She really kind of looks like my cousin."
There's more than one joke at play in this scene, even going beyond the nod to Routh having previously played Supergirl's cousin, Superman. Felicity's comments hung a lampshade on comments among the fandom that Felicity and Kara had similar looks and personalities. It also poked a little bit of fun at Ray, who had been romantically involved with Felicity for a time, and suggested an unsavory reason for why he had been initially attracted to her. The implications were not lost on Felicity, who gave Ray an odd look in response to his declaration.
Regardless of the intent of the Invasion! tease, the idea of Routh playing multiple heroes in the Arrowverse is not without precedent. The Elseworlds crossover event in 2018 established that the reality of the 1990 The Flash series was set on Earth-90. This revelation left actor John Wesley Shipp playing two Arrowverse superheroes - The Flash of Earth-90, whose real name is Barry Allen, and The Flash of Earth 3, Jay Garrick. So there is nothing to stop Brandon Routh from playing both The Atom and Superman. It remains to be seen, however, if any characters from Birds of Prey, Smallville or the DC Universe shows might cameo as well.