Warning: SPOILERS ahead for Supergirl: Season 3 Episode 1
Supergirl spent its second season (the first on its new home on The CW) introducing new characters and building up a new status quo that put it, for better or worse, more in line with the tone of fellow DCEU TV series like Arrow, Flash and Legends of Tomorrow. Some of those changes were widely mourned (Supergirl fans have yet to "get over" the loss of Calista Flockhart's Cat Grant) while others, like Alex Danvers' coming out story and romance with Maggie Sawyer were near-universally praised. But the most divisive was the presence of Mon-El, the alien visitor who became Kara's superhero-trainee and eventual love interest.
While the relationship was a hit with some fans, many others felt that having so much of Kara's story revolving around a boyfriend (Mon-El's evil mother ultimately turned out to be the season-ending Big Bad) was a betrayal of the "independent millennial working-girl" setup that had drawn them to the series in the first place.
Mon-El's story itself, of course, ended with the Daxamite prince being forced to depart Earth for good; the downside of a bio-weapon that helped dispose of his mother's forces by making Earth's atmosphere toxic to their species. But if fans who had never warmed to the character or his effect on Supergirl herself were hoping that meant season 3 would be moving on from him entirely, the season premiere was probably something of a disappointment. Mon-El is gone, but his presence is all over the series' new status quo - everything has been shaken up, and it's got a lot to do with Kara having turned into a distant, all-business superhero workhorse who has time for saving the city and basically nothing else. It's all couched in terms of Kara wanting to put her human identity on the back-burner and re-embrace her alien side, but everyone can see the real issue is she's still not "over" how her relationship ended (who would be?)
Still, even if one was "onboard" with Mon-El in season 2 and sad to see him go, it's hard not to be just a little bit off-put by his presence still being such a factor a full season-break later. Obviously, it would be unrealistic to depict Supergirl being fully recovered from what happened, but seeding "sad because of Mon-El" as the basis for almost every change to the status-quo of Kara's relationships right off the bat almost feels like an early limiting of where her story can go for the rest of season 3. If this one specific hangup is going to be blamed for her distance from Lena Luthor, her shirking work at CatCo, her more aggressive approach to heroics and her drawing back from sister-time with Alex, that either means all of those threads can be solved at once or that they can be drawn-out across the entire season - neither option seeming particularly appealing at this point.
On the other hand, "The Girl of Steel" did appear to at least suggest that Kara's happy(er) state of mind and nominal re-engagement with friends and family at the episode's end could be (if the series' creators so choose) where the Mon-El thread gets left for now. There are certainly no shortage of other threads being set in motion: Lena Luthor has placed herself in charge of CatCo (Cat Grant having departed, once again, to become President Marsden's press secretary - a plot device that seems designed to let Flockhart literally phone-in her cameos), which should have the characters working together in a way sure to get "Super/Corp" shippers riled-up all over again. There's a new corporate-level villain in Adrian Pasdar's Morgan Edge guaranteed to spur fan-speculation (in the comics, a frontman for the Apokolips-adjacent crime syndicate "Intergang.") And expected Season 3 main-villain "Reign" even made an early appearance in a decidedly non-villainous context.
That's probably the right track to take - at least assuming that Mon-El really is gone from the narrative for the forseeable future. The fact that he wasn't fully "killed off" at the end of Season 2 indicates that the writers more likely than not have further plans for him, but if he's going to be off-world for a long stretch it would be a mistake to keep bringing him up as though nothing else could possibly be on Supergirl's mind amid all this new intrigue (to say nothing of inviting no shortage of "Where's Poochie?" jokes from the fanbase.) This is, clearly, a plot thread that will need to be resolved sooner than later; but that doesn't mean it has to dominate the season going forward.