"We Can Be Heroes" may have been primarily pitched as the big return of Season 1 villainess Livewire, but the episode's central image had little to do with the electricity-throwing nemesis: Supergirl standing alone in the middle of the DEO lobby as the odd woman out, while her coupled-up friends and associates departed to go about their own coupled-up business. But just as the visual cues were driving the point of its heroine's infrequent spells of loneliness, the overarching plot is clearly building to a big change in that equation.
If superhero stories have a favorite "melancholy" mode, it's in reminding us that it's lonely at the top. It comes up less often with members of the Superman family than it does for, say, Batman or Spider-Man (for whom grousing about their own self-imposed social-isolation is practically a secondary calling); but read/watch a Superman or Supergirl story long enough and you'll eventually be reminded that sometimes it kind of sucks to be The Last _____ Of Krypton - no matter how many surrogate families you build up around yourself.
In Supergirl's case, her own singularity is usually mitigated by the way her tendency towards team-player status (closeness with her sister, devotion to CatCo, joining The D.E.O.) keeps her suitably occupied. But "We Can Be Heroes" conspires to leave her with little to do but face her limited social options as events place her paired-off supporting cast in their own storylines to a greater degree than typical for the series: As the narrative wraps up, Kara is the only one without a playmate to hurry off with.
For those needing a refresher, said couplings include Alex Danvers and Maggie Sawyer (now a happily-functioning romantic duo hopscotching through their Honeymoon Phase), Miss Martian and Martian Manhunter (gradually moving from inter-species disdain to something else) and platonic BFF's James Olsen and Jason Winn - who clash over revealing their "Guardian" vigilante operation to Supergirl, but find themselves more unified than ever when she refuses to support their dangerous do-gooding hobby.
That's not to say that Kara doesn't at least have prospects of her own. We learn that Livewire (thought to have escaped from prison but in reality abducted by a mad scientist looking to siphon off her electrical abilities) has transferred her revenge-fixation from the now-departed Cat Grant to Supergirl herself - who appears particularly excited/annoyed to have a recurring "nemesis" like her cousin's various foes. Whether or not the series actually plans for her to actually become Supergirl's repeat-problem remains to be seen, though DC has been hot to push the character (conceived as a Superman-family answer to Harley Quinn) since she was created for the animated series back in the 90s.
More substantively, though, the episode moved ahead the storyline for Kara's nearest possible potential romantic interest, Mon-El, who (having declared his intent to join Kara in the superhero game last episode) graduates here to field work with mixed results, but a rare moment of unguarded honesty. He admits that his increased desire to better himself has less to do with a growing sense of heroic responsibility, and more to do with being sweet on Kara. As the episode leaves off, however, she's still claiming to not be interested - meaning that being Earth's Strongest Third Wheel among almost all of her significant friendships is now, at least in part, a condition of Kara's own making.
Realistically, Kara and Mon-El becoming at least some sort of an "item" is probably inevitable. The jealousy that it drives in James and Winn is too key to their story (sometimes to an uncomfortable degree - "we're both depressed about not getting the same girl" is an odd basis for a friendship) to not push all the way forward, and the impending reveal of Mon-El's remaining "dark secret" about his origins will have that much more impact in the context of a relationship storyline.
It's not likely to end well (Mon-El will get his super-suit and we'll get some team-up adventures, but fans were sold on a series about Supergirl coming into her own, not a superhero couple show), but it'll also likely stick around long enough to allow Kara to finally have an adult relationship that doesn't leave audiences with too many questions to dwell on regarding how kryptonian/human sex is supposed to "work" mechanically considering the extreme strength-differential. There's probably some narrative risk invovled in that in terms of Mon-El's perception as a character (i.e. "Oh, so that guy was only ever here so Kara could finally get a super-hookup?"), but the calculation is likely that it's worth the risk: So much of Supergirl is about relationship-dynamics that it'd be almost improbably discordant for the title character to still only ever get schoolgirl crushes or awkward flirtations.
Wherever the Kara/Mon-El storyline is going, fans likely won't be waiting too much longer to find out: Valentines Day is only two weeks away.