Supergirl doesn't do "subtle." Even before its move from CBS to The CW and open embrace of the Arrowverse saturation-bombing approach to superhero genre staples, it's been a series that takes it's basic premise of re-staging Superman and Superboy story-setups from the Gold and Silver Age of comics as a female Millennial coming-of-age drama as license to play things broad and arch from the very beginning. It doesn't simply telegraph it's big story beats or the goal of its character arcs, it points to the outfield like Babe Ruth calling his home run shot - practically begging the audience to judge it on how well it hits the point everyone already knows it's aiming for.
Season 3, with the exception of an earlier-than-customary crossover with the rest of the Arrowverse (last week's well-received "Crisis on Earth X"), has largely concerned itself with two - or, depending on how you frame it, possibly three - running main storylines. On the civilian-life side, the Danvers sisters are jointly grappling with romances that collapsed under deeply unfair circumstances - Kara having lost Mon-El when he was forced to depart the Earth in order to thwart an alien invasion by his evil mother, while Alex was forced to end her engagement to fiancée Maggie Sawyer when they couldn't reconcile their different outlooks on having children (Alex was a "yes.") On the superhero side of things, it's been all about newcomer Odette Annable's Samantha Arias discovering that she's actually a Kryptonian bio-weapon called "Reign."
Related: How Supergirl Introduced The Legion of Super-Heroes
With the series now hitting its Winter break period, both storylines have come to a head in adjacently-dramatic fashion: No sooner had both sisters begun to claw back to life-goes-on territory (Alex even perked up enough for a one night stand with White Canary during the Crisis sojourn) than did Mon-El reappear to accidentally break Kara's heart all over again with the reveal that he'd A) wound up stuck in the 31st Century for 7 years and B) eventually fallen for and married Imra "Saturn Girl" Deen. Meanwhile, Samantha mistakenly went looking to unlock her Reign powers under the assumption that they were meant for good - setting her up to utterly wreck an already strained relationship with her terminally-neglected daughter Ruby.
As of Episode 10, "Reign," all of that has gotten a touch more urgent: Now able to fully transform into Reign (which apparently involves donning an all-black answer to Supergirl's uniform and a somewhat silly-looking masquerade mask straight out of Eyes Wide Shut) but unable to remember having done so, Sam is making things even worse with Ruby - who was already operating at "endanger-my-life-to-see-if-mom-is-actually-a-superhero" levels of neediness. Reign's mission: Nothing less than the embodiment of a demonic "Worldkiller" entity from Krypton's polytheistic ancient past, she's here to kill "sinners" and engage Kara in what are easily the most intensely-executed fight scenes the series has dropped her into outside of the crossovers; including a brutal close-out beatdown that felt a lot like an attempt to prove it could outgun Batman V Superman's titular slugfest the same way "Crisis" was praised as a superior answer to Justice League.
On the relationship end, Mon-El and Imra can't seem to find anything to say to Kara that doesn't make things worse ("We based our future superhero team on how cool Mon-El said his ex was, and I'm your biggest fan!" certainly didn't do it); leaving her all too willing to take Alex's advice to fight Reign in full-on "I'm a cold inhuman alien" mode... which ends with her thoroughly beaten-up and fighting for life as the requisite Winter Break cliffhanger (Gee, wonder what her lesson will be?) But the two storylines have also been building to an intersection on Alex's end: She's increasingly found herself making friends with Ruby when their respective super-relatives are called away "for work."
Again, subtlety is not one of Supergirl's yellow-sun-enhanced powers: It's not hard to see the trajectory of a precocious kid with a (probably) doomed unwitting-supervillain mom entering the plot around the same time that one of the main recurring characters has been struck with mommy-fever. The relative ease with which a loose "Alex adopts Ruby after Reign is killed/imprisoned/etc" arc would fall into place for these characters almost makes one want to assume that there's some epic misdirection happening - but that would almost certainly put a wrench in the equally-inevitable "Kara reconnects with her humanity to help Reign reconnect with hers" arc happening on the other end of the ledger (unless we're heading for some sort of swerve with one or more new-season characters following Mon-El, Imra and The Legion of Superheroes back to the year 3000 for some reason, of course.)
If the former, it would be in keeping with Supergirl's tendency to keep it's supporting characters repeating variations on their foundational arcs; with Alex's Season 1 "revealing I'm a secret agent" arc segueing to Season 2's "revealing I'm a lesbian" to Season 3's "revealing I always wanted kids" complete with happy-with-an-asterisk ending. Some of it may depend on whether or not fans are open to a recurring kid character joining the cast - in the past, Supergirl has ably navigated the space between fan-appeal superhero indulgence and more conventional network genre fare; but adding child actors is often seen as a serious "Jump the shark" moment by TV-appraisers who still use "Jump the shark" as a colloquialism. If this is the direction things are headed, it will come down to the stories being told - and how Supergirl aims to tell them.