WARNING: The following contains spoilers for Stargirl Season 2, Episode 1, "Summer School: Part One," which aired Tuesday on the CW.
Ever since it was announced, Stargirl was always meant to be separate from the CW's Arrowverse shows. This was to allow its Justice Society superhero legacy to be unencumbered by the happenings of other superheroes, not to mention the obvious budgetary differences. However, the two universes may finally be coming together.
Connecting the two universes may be the dastardly villain Per Degaton, a Justice Society foe who's known to travel through time in the comic books. Per Degaton's already shown up in the Arrowverse, and this new version and the Flash could be connected to the one mentioned on Stargirl. Here's how the Scarlet Speedster and a scarlet-maned chrono-fiend could be what finally opens up the new Arrowverse Multiverse.
The Flash and Per Degaton
In the first episode of Stargirl's second season, Pat and Courtney briefly discuss some of the Justice Society's old villains in the Injustice Society. One of these foes is Per Degaton, who Courtney asks Pat about. Pat states that Per Degaton had been beaten by the Flash and subsequently banished to another dimension. This opens up all kinds of possibilities for the show, as well as the still unrelated Arrowverse.
For one, it would seem that Pat is already aware of the Multiverse. This means that the previous generation of superheroes on this Earth also knew about it, with this Post-Crisis knowledge putting them ahead of the game compared to the denizens of the Arrowverse. The fact that the Flash is the one who banished Per Degaton away is also an interesting choice, and it further opens the door to uniting Stargirl and the Arrowverse shows.
Legends of Tomorrow
Per Degaton previously appeared in the Arrowverse, showing up as a young boy who's trained to be the protege of Vandal Savage. The show's time-traveling premise reflected the character from the comics, but beyond this, as well as his trademark black clothing and red hair, the character bore little resemblance to the comic book version's ambitions. With Stargirl, however, there's a chance that this version could become a much bigger threat.
As mentioned, the Per Degaton of Stargirl's world was banished to another dimension. It's possible that this boy is that same Per Degaton, changed into a child with no memory of his previous life due to dimension-hopping properties of some sort. Upon learning this, he could become a major villain in a crossover event involving the Arrowverse and Stargirl, finally alerting the Arrowverse heroes to the fact that the Multiverse still exists.
The Flash is also known for traveling to different time periods and dimensions, so the banishing of Per Degaton makes sense. Likewise, The Flash on Stargirl will be the Justice Society's Jay Garrick, who's played by none other than John Wesley Shipp. Shipp, of course, played Barry Allen in the '90s live-action The Flash series, but he also played Henry Allen, Barry's father in the Arrowverse Flash show and Jay Garrick in its version of Earth-3. Stargirl's world would seem to be the new Earth-2, especially given its Justice Society connection, which makes using Garrick and Per Degaton as the multiversal connecting points an even stronger idea.
To see how Stargirl lays the groundwork for an Arrowverse crossover, new episodes air Tuesdays on The CW.