Ever since Supergirl was first announced, fans have been hoping that eventually this show would be incorporated into the CW's DC multiverse. Although Kara's Earth is different from that of The Flash and Arrow, the multiverse's introduction in The Flash Season 2 allowed for Barry to cross over into Kara's world. This episode was a rousing success, only fueling the desire for Supergirl to be incorporated into the Flarrowverse more.
Now our prayers have been answered — it was recently announced that not only will Supergirl live on the CW from now on, there's a massive crossover planned later this year. This will bring together The Flash, Arrow, Supergirl, and Legends of Tomorrow. And it's gonna be awesome.
Obviously, the plot possibilities for this are endless (one might even say... infinite), but there's one arc which the creators have been teasing since The Flash Season 1, and it might just be time for a crisis.
Destruction Of The Multiverse
Because of its impressive 50 year plus run, DC has multiple continuities which can be quite confusing. There have been several canon resets to try and organize these into one coherent continuity. When it comes to the multiverse, the most important resets are Crisis On Infinite Earths (1985) and Infinite Crisis (2005).
Both of these plotlines are very complex, but the upshot of it is that Crisis On Infinite Earths combined the multiverse into just one Earth, effectively removing Earth-2 and other Earths from the canon.
Infinite Crisis reintroduced survivors from Earth-2, including Power Girl's origins as the Earth-2 Supergirl, and restored the multiverse. Supergirl has a big role in both the comic Crisis arcs — Earth-1 Kara sacrifices herself in Crisis On Infinite Earths, and of course Earth-2 Kara was marooned on Earth-1, becoming Power Girl.
Back in The Flash Season 1, it seemed as though the creators were teasing a Crisis arc, thanks to the newspaper from the future which flashed up frequently throughout the season. Of course, this "crisis" is set in 2024, but with Barry's ability to time travel, not to mention Rip Hunter's time ship the Waverider, nothing is off limits. And while we're on the subject of Legends of Tomorrow, it's worth mentioning that the finale introduced Hourman, a member of the Justice Society.
This is a team best known for being the Earth-2's equivalent to the Justice League, and Hourman's arrival implies that Season 2 of Legends will start to explore the multiverse too.
Could We See Power Girl's Origins?
It could be that the CW are building towards a big Crisis event, which could help to incorporate Supergirl into the same universe as the rest of the CW shows. Perhaps the crossover even will feature a plot where the multiverse starts crashing together, with the consequence of Kara's world and Barry's merging. This could all connect back to the Flash from the future vanishing in that "crisis".
And it might be that the Kara we know and love could eventually become marooned on Earth-1, following the Power Girl arc from the comics. Or maybe the CW are planning something completely different.
There's also the possibility that we could see another Kara on Earth-1 or even Earth-2 — although it was revealed in "Welcome To Earth-Two" that this version of Barry knows Bruce Wayne and Diana Prince, the other member of DC's Holy Trinity was conspicuously absent. Are Clark and Kara hiding on Earth-1 or 2 somewhere, or have they just not arrived yet?
Ok, we can fall down the rabbit hole of speculation when we think about what the upcoming crossovers could mean. There's Arrow and Legends of Tomorrow to take into account too — both shows have vast mythologies and their own crises to contend with. Suffice to say, the plot possibilities are very exciting, and it'll be really interesting to see how it all plays out.