With the recent announcement that Supergirl is ending in 2021, Stargirl will become essential to the Arrowverse moving forward. Though the series hasn't been properly integrated into the franchise yet, the former DC Universe original series made a cameo appearance in last season's Crisis of Infinite Earths crossover. The upcoming second season of Stargirl and the eventual Arrowverse crossover between it and the franchise's remaining shows will be an important step in incorporating Stargirl into the shared universe and expanding the scope of the live-action TV universe.
The second season of Stargirl will deal with the fallout of the Justice Society of America's hard-fought victory against the Injustice Society of America. The ISA was dealt a huge blow when Dragon King, Brainwave and Icicle died in battle. Yet, the villainous group is still a threat because Cindy has found the crystal containing Eclipso and former ISA member The Shade is still around. However, the biggest threat to the JSA in Season 2 might be someone claiming to be Starman.
While the Whitmore-Dugan family were celebrating Christmas, a man claiming to be Pat Dugan's old partner, Sylvester Pemberton, knocked on the door of Pat's former residence. Starman supposedly died during a battle with the ISA in 2010 when Icicle stabbed him with an ice dagger. If this man is actually the real Pemberton, it wouldn't make sense for him to lie about his whereabouts to his close friend Pat. On the other hand, Icicle's dagger may have placed him into a coma that he only recently awakened from once the villain died. And while this seems like a plausible explanation for Pemberton's sudden reappearance, the man claiming to be him might actually be a doppelganger from an alternate universe.
A similar situation occurred on The Flash when a man claiming to be Jay Garrick was actually revealed to be Hunter Zolomon, a villain from Earth-2. Strangely enough, Hunter's original universe was destroyed during Crisis on Infinite Earths and was replaced with a new version of Earth-2, which is the setting of Stargirl. If the person claiming to be Pemberton is actually from an alternate universe, he could be an evil doppelganger from Earth-3, the universe that usually contains evil counterparts in the comics. This twist could be a revelation for the JSA and could potentially set up a crossover with the Arrowverse heroes on Earth-Prime.
In hindsight, Stargirl probably should have been fused into Earth-Prime during the Crisis event now that the DC Universe streaming service is transitioning from a streaming TV service into a digital-comics subscription. The other DC Universe original shows are moving to HBO Max, while Stargirl and Swamp Thing will air exclusively on The CW. And though an actual crossover between the Arrowverse heroes seems ever more likely now that they're on the same network, integrating the series into the universe is still going be difficult.
Supergirl originally ran on CBS before it was picked up by The CW, and despite that show being on the same network as Arrow and The Flash, the series was placed in an alternate universe to explain its separation. Aside from the annual crossover, it took years and a massive continuity-changing event to fuse those universes into one cohesive Earth, and now that it's finally happened, the series is ending. The fusion of these universes didn't come without any problems, as in this new Post-Crisis universe, it's hard to determine what events from the series actually unfolded in Supergirl's new continuity.
Likewise, Stargirl has its own continuity with a legacy of superheroes that stretches back for decades with teams like the All-Star Squadron and the Freedom Fighters. It may be too early to integrate the series into Earth-Prime as that could require retconning all that established history and lore. Going forward, Stargirl has to be careful when it comes to joining the Arrowverse so it doesn't lose its unique identity. Season 2 of the series is going to be a litmus test for how the series can continue to unfold in the future -- and how it could become a pillar for the Arrowverse.
DC Universe's Stargirl stars Brec Bassinger as Courtney Whitmore (Stargirl) and Luke Wilson as Pat Dugan (S.T.R.I.P.E.). It also stars Amy Smart, Yvette Monreal, Anjelika Washington, Cameron Gellman, Trae Romano, Jake Austin Walker, Hunter Sansone, Meg DeLacy, Neil Jackson and Christopher James Baker. The series will return for Season 2 on The CW.