Supergirl takes place in an alternate universe from the rest of the CW superhero series, a leftover element from its originally having debuted on the competing CBS network before moving for Season 2. However, now that the superheroine has participated in the massive network-wide weeklong "Invasion!" crossover, her place as part of the so-called "Arrowverse" is more secure than ever. Now, according to the series producer, even greater crossover possibilities have opened up.
"Invasion" has revealed the existence of Supergirl's alternate universe and Kara Zor-El herself (previously only encountered by The Flash) to the rest of the Arrowverse heroes, while also alerting Kara herself to the presence of interdimensional, time travel and alien threats like The Dominators. But according to producer Andrew Kreisberg, things could easily become more involved going forward -- though what that could potentially mean seems to still be very much up in the air.
"We hadn't discussed it formally, but I think that certainly after these three episodes, there's literally no limit to the things that we could do. There's still an endless treasure trove of DC Comics characters, and stories, and titles that we haven't even come close to touching yet. We're certainly not wanting for ideas for the future. We know that they're out there for us."
While this comment stops well short of actually confirming Power Girl for an upcoming CW appearance, it's easy to see why fans have been predicting her eventual arrival ever since Supergirl began airing. Originally hailing from DC's pre-Crisis On Infinite Earths "Earth 2" continuinity (where the lives and identities of heroes mirrored the WWII-era "Golden Age" DC heroes), Power Girl was an older, more violence-prone incarnation of Supergirl herself who had changed her superheroine codename as a statement of independence.
Thus far, no offficial "dopplegangers" of heroes from the main Earth of Arrow and the other CW series have been shown to exist on Supergirl's now-designated "Earth 38," though the idea has cropped up amid the dimension-hopping episodes of The Flash and the recent "Flashpoint" storylines. Such crossovers have now become a regular annual feature of The CW's yearly schedule, likely meaning that Supergirl will see more of her newfound comrades in later seasons (assuming her series is granted a renewal).
Supergirl airs Mondays @8pm on The CW.