The CW's Supergirl has taken the Superman canon and turned it on its head, giving it a whole new life, but that means the show totally leaves us guessing when it comes to, well, a lot. There's still so much we don't know about the show's badass superheroine and her Kryptonian heritage, but that's just the tip of the iceberg – that's also why so many fan theories about Supergirl and its characters have popped up over the years.
Are Supergirl and Smallville parallel universes? Is Lena Luthor really a good person? What's the deal with Mon-El? How does Batman fit into the CW show? Will Saturn Girl die in order to make way for Karamel once again?
There are plenty of mysteries on Supergirl, which means there's a dedicated group of fans trying to solve them. Some Supergirl fan theories are a bit far out – like one about Saturn Girl using mind control on Mon-El in order to keep their marriage going – while others make perfect sense.
According to Redditor /u/meagancarter4, it's quite possible that the Man of Steel had already met Mon-El sometime in the future before the events of Seasons 2 and 3. When Kara visits the Fortress of Solitude in Season 1, it's revealed that Superman owns a Legion flight ring and has it on display in his HQ. This would seem to indicate that Superman has already crossed paths with the Legion – and Mon-El – by the time Kara visits the Fortress for the first time.
In Season 2, when Superman first "meets" Mon-El in the present day, the Man of Steel might actually be cracking a joke when he says Kara's boyfriend "must be a great guy." By this point, Superman would know for a fact that Mon-El is a good guy because they've gone on many adventures in the 25th century.
There's always been more to the Legion's arrival in Kara's time than meets the eye. In fact, Redditor /u/parduscat has theorized that the Legion has gone back in time to stop Kara from being killed by the Worldkillers – or becoming one herself. The logic is simple: if the Earth suffered a cataclysmic event in the 25th century, one can assume that it was at the Worldkillers' hands. This means that Kara was unable to defeat the Worldkillers on her own in the present day, which is why the Legion has come to help her out, ultimately altering the timeline in the process.
In season 3 of Supergirl, Kara is tasked with chasing down a MacGuffin, a black rock that goes by the name, Harun-El. Unfortunately for her, when she eventually finds it, she discovers that the rock has been allowing Kryptonians to live and breathe inside Argo city. Taking the entire rock would be devastating for the Kryptonians, so she decides to take only a small piece of the rock. According to Redditor /u/the3dtom, there might be more to this rock than meets the eye.
"Kara touched it, therefore creating a second Kara (at the end of tonight's finale). Thoughts? Maybe they combined the Red Son storyline with the Black Kryptonite?"
In the comics, Black Kryptonite actually splits Supergirl into two distinct personalities. The Red Son comic sees Superman land smack dab in the middle of Soviet Russia, rather than his traditional home of Kansas. It would definitely be interesting to see Kara split into both an evil and good version of her heroic persona, especially if it follows the comics and she ends up in Siberia.
Lots of fans really want to see Smallville star Tom Welling make an appearance on Supergirl, and they're willing to devise any crazy theory to make it happen. Redditor /u/soulxhawk suggests that there are so many similarities between Kara's story and Smallville Clark's that they must actually exist in parallel universes that potentially mirror each other. It turns out Clark and Kara's respective universes have quite a lot in common, including the protagonists' friendships with Luthors and their mentorships under Martian Manhunter.
One more interesting bit: it's already been shown that the Flash can travel between different Earths, as he did when he found himself face to face with Supergirl in Season 1. Could he one day arrive on Smallville's Earth and bring Tom Welling to the Arrowverse?