Superman isn't just joining the next Arrowverse crossover event, there will be some "epic" interactions between the Man of Steel, Stephen Amell's Green Arrow and Grant Gustin's The Flash. So far in the Arrowverse Tyler Hoechlin's Superman has been contained solely to Supergirl and its home universe of Earth-38. However the next Arrowverse crossover will bring Superman (and Lois Lane) into the larger Arrow multiverse.
Superman's appearance won't be a superficial cameo either. Despite not being seen since the end of Supergirl season 2, Hoechlin's Supes will be a significant figure in all three parts of the annual crossover. The long-awaited team-up between Superman, Green Arrow and The Flash is finally arriving.
In an interview with EW, Flash star Grant Gustin gave his thoughts on Superman joining the crossover. Gustin, who is a longtime Superman fan, is ecstatic over getting to share the screen with the Big Blue Boy Scout. Gustin explained:
To see Flash and Superman together and to be one of the guys in the suits is something I’m definitely pretty excited about. It did feel like something they would always save for the features, to be honest. But I feel like the Arrowverse has kind of been changing that stigma with the whole TV is the lesser medium. I think it kind of doesn’t matter these days with the streaming and content’s kind of everywhere. So, it is cool to see us kind of rise to even another level and bring all of us together for these crossovers. It’s pretty epic.
As for what those "pretty epic" Superman, Green Arrow and Flash moments will entail, that can only be guessed. Very little is known about the 2018 Arrowverse crossover other than a portion of it will take place in Gotham City and it'll be Ruby Rose's debut as Batwoman. It stands to reason, though, that the crossover will feature a rather dire world-shattering event. Superman has sat out two of the previous Arrowverse crossovers, which involved an Alien invasion and the arrival of alternate universe Nazi invaders. Granted both invasions took place on Earth-1, not Earth-38, but Supergirl's cousin would've been helpful in either situation. It's possible that 2018 crossover's will be the most cataclysmic one yet.
Perhaps, and this is pure speculation, the 2018 crossover will see Supergirl captured or incapacitated in some way. In the Arrowverse, Supergirl is stronger than Superman but if something were to happen to Kara Danvers, Superman is a pretty good replacement. If Supergirl isn't put into danger, maybe her world will be threatened instead. For obvious reasons, Earth-1 has been treated as the primary world of the Arrowverse. It would be interesting to switch things up and see Earth-38 be under siege for once, requiring Kara to call upon Barry Allen, Oliver Queen and her cousin to save the universe.
Regardless of how Superman factors into the crossover, his existence is a big step forward for the Arrowverse. Superman has been one hero in DC Comics lore that's almost always been isolated, outside of the movies and comics. Superman is one of the faces of the Justice League in the DCEU but the TV world has treated him as a supporting player. Supergirl is the (rightful) Kryptonian star, while Superman is off having his own off-screen adventures. Superman joining the 2018 crossover and having evidently so much interaction with Green Arrow and Flash breaks down that wall at last. There are still no plans for Batman to join the Arrowverse, but Superman could play a much larger role in the Arrowverse in the future. The chances of a full Superman spinoff are slim, but he could become a recurring member of the Arrowverse crossovers moving forward.