As most of the various Arrowverse shows have returned from their midseason breaks this month, and thus transferred into the new year, one thing on most fans minds is what the crossover of 2018 will be.
Now that all the shows are interlinked -- save for Black Lightning, of course, though that could happen later -- the real fun of the crossovers is more about just watching these characters that we know and love bounce off the characters from sister shows for a few hours. 2017's "Crisis on Earth-X" realized this, and in turn delivered some of the best character moments that have ever existed in the whole enterprise.
To date, however, each of the crossovers have taken place on Earth-1, even after Supergirl migrated over to CW. If the network wants to keep things moving along properly as the crossover get bigger and better every year, there's one way to do it: base 2018's crossover on Earth-38, Supergirl's world.
Because Supergirl originally aired on CBS, it had to take place in a separate universe from the CW's DCTV shows. The only time an Arrowverse character interacted with her world in her first season was when the Flash quite literally ran into her universe and hung around for a few days. Even now that she's officially a part of The CW's line-up, it's made sense to have her jump to Earth-1 for crossovers. After all, compared to the two dozen or so cast members that encompass Arrow, Flash, and Legends of Tomorrow, it's much easier to bring Kara and an extra friend over to the more populated universe than the other way around.
Still, since "Crisis" showed that alternate realities are certainly fair game for these mega-events, it's only logical to have the next yearly superhero melee centered around Kara's neck of the woods. In 2016's "Invasion!" event, Barry promised Kara that if she ever needed help from him or any of the other Arrowverse characters, all she had to do was hop over to their universe and give them a ring. Nothing's come of that yet, which will make the moment that it does happen all the more important. Even with two Kryptonians in Supergirl's universe, there are a multitude of threats that would require Kara to send out the call to Teams Arrow, Flash and Legends for some additional metahuman firepower.