WARNING: This following contains spoilers for Superman & Lois Season 1.
Superman & Lois has been a breath of fresh air among the expansive Arrowverse, boasting production values far beyond those other shows and feeling right at home with HBO Max's programming. Given this incredible success, a second season is obviously on the way, and it should take advantage of the show's triumphs so far to expand Superman's family.
The show focuses not only on the Couple of Steel but also on their two teenage boys. However, most notably missing is Superman's cousin Kara. Various reasons may see Kara Zor-El remain absent on Superman & Lois for a while, but another version of Supergirl could and should show up. Here's how Superman's family could see a new Maiden of Might debut in the Arrowverse.
Supergirl's Absence From Superman & Lois
Superman & Lois took advantage of its Post-Crisis setup and essentially removed itself from the Arrowverse, having as little to do with the other shows as possible. This allowed it to establish its own stakes and status quo while also showing off budgets and scopes quite beyond the other CW DC shows. Fans, however, noted how there was not even a reference to Superman's cousin Supergirl in the first season, let alone an appearance.
This could be due to a number of factors, most notably that the pandemic has affected filming and actor travel. Likewise, Supergirl is in its final season, with Kara actor Melissa Benoist moving on from the character. It's still unknown how her show will end, but it may leave Kara in a much different status quo, such as living in the future with the Legion of Superheroes, that removes her from the goings-on of the present-day Arrowverse. In the likely event that Kara leaves the present, a new Supergirl can take her place. While Kara Zor-El is easily the most popular Supergirl, she's not the only young woman to wear the S on her chest, leaving the door open for a new Girl of Steel.
Contenders for Superman & Lois' New Supergirl
Perhaps the most obvious Kara replacement would be the Cir-El version of Supergirl. This was a Post-Crisis version of the character introduced during a time when DC editorial was hesitant to have any other prominent Kryptonians besides Superman, making him truly the Last Son. This saw Kara Zor-El go without being reintroduced for years, with Cir-El being Superman and Lois' supposed daughter from the future. This turned out not to be the case, but the character did have a sizable role as a Metropolis hero during her brief existence.
One thing keeping Cir-El from showing up in the Arrowverse is that the Supergirl showing up in the theatrical Flash movie is seemingly based on her. Another idea might be the angelic Supergirl that appeared in Peter David's run on the character. Though she wore the familiar blue suit, she had a completely different power set and ties to the literal Biblical heaven. This aspect might keep her from being considered, especially since this was an even more egregious example of DC having a Supergirl with no relation whatsoever to Superman.
There's also the Matrix Supergirl, who was a protoplasmic being created by an alternate universe version of Lex Luthor. Though she still wasn't Kryptonian, her powers of imitation were facsimiles of Superman's. Perhaps the biggest left-field idea would be to emulate the classic Earth-2 Superman mythology and have Lois and Clark adopt another version of Kara. Instead of Supergirl, she'd grow up to be Power Girl, establishing an identity quite different from that of the normal Kara Zor-El.
Some of these may be stretches, but the void of Supergirl should still be filled if the "classic" Kara is no longer around. She's clearly established a legacy in the Arrowverse, both in National City and the universe in general. Likewise, it's obvious that the void presents the opportunity to bring in more obscure versions of Supergirl than the one usually seen in and outside of the comics. Having a female Kryptonian, especially one who's the child of Lois and Clark, would also bring in a new dynamic given that their house has two rambunctious boys. Especially given that she (through a crossover with The Flash) was the Arrowverse's segway into the Superman mythos, Supergirl's enduring presence should stick around in some way, even if her show ends.
Superman & Lois: The Complete First Season arrives on Blu-ray and DVD on Oct. 19.