The CW's DC universe is currently comprised of four superhero shows. Arrow, The Flash, and Legends of Tomorrow are all set on Earth-1, but the Supergirl action all takes place over on Earth-38, where aliens live openly among the human population, Lynda Carter is the president, and all a superhero needs to conceal her secret identity is a snazzy pair of spectacles. The Flash has shown different Earths usually involve doppelgangers of certain characters, so a big question has been if and when we'll see any Earth-38 versions of Arrow-verse characters. Supergirl executive producer Andrew Kreisberg come out and explained why the show hasn't explored Arrow-verse doppelgangers on Earth-38, saying this:
We've seen Kara interacting with some of our familiar Earth-1 superheroes off and on during the first two seasons of Supergirl, but the heroes really only join forces for big crossover events. If we regularly saw Kara interacting with Barry Allen or Oliver Queen or Sara Lance, some of the crossover magic might be lost, even if they were simply Earth-38 versions of the characters. Andrew Kreisberg's reveal that the Supergirl team is avoiding doppelgangers to keep from spiraling out of control indicates that we can count on the show keeping its focus on Kara and Co. After all, there's already plenty of wackiness on Supergirl without bringing in different versions of Barry and Oliver. We have shape-shifting Martians, a Daxamite with a dark past, and a heroine who can be turned evil when exposed to a certain kind of Kryptonite. Who needs doppelgangers anyway?
That said, Andrew Kreisberg won't necessarily rule out the possibility of someday finding a reason to introduce the Earth-38 incarnations of Arrow-verse characters. He also mentioned this in his chat with IGN:
Thanks to The Flash, we do know that some Earths are shockingly similar to each other. In fact, when Barry and his pals were missing having a Harrison Wells on the team in the beginning of Season 3, they simply fished a different version out of Earth-19. It would stand to reason that there are versions of Arrow-verse characters milling around on Earth-38; we just haven't seen them. Who knows? Maybe instead of introducing Arrow-verse doubles on Supergirl, we could get some Supergirl doubles on Arrow, The Flash, and Legends of Tomorrow.
Personally, I'm a huge proponent of keeping Supergirl mostly separate from the shenanigans on Earth-1, but I'd be game to see Earth-38's Harrison Wells emerge from the woodwork at some point. Wells isn't such a major character that he'd upset anything on Supergirl the way Barry or Oliver would, and the many versions of Wells are practically a running joke in the Arrow-verse.
Tune in to The CW on Mondays at 8 p.m. ET to see what's next for Kara on Supergirl, and don't forget to take a gander at our midseason TV premiere schedule.