Season 3 of Supergirl has introduced viewers to a whole new super-team: the Legion of Super-Heroes. Originating in the 30th century, this band of aliens and metahumans have proven to be tremendous allies in the fight against Reign. However, their arc is clearly setting up for the Legion to ultimately go back into cryogenic sleep, and continue their journey back to their own time.That probably means the Legion's days in Supergirl are effectively numbered. What's more, it's clear that time-travel in Supergirl isn't quite as easy as it is in the Arrowverse. So once they're gone, the Legion are unlikely to come back any time soon. What's next for the Legion?

The Legion of Super Heroes

The Legion of Super Heroes are one of DC's most famous super-teams. In fact, they actually predate the Justice League by two years! They were originally envisioned as supporting characters for Superboy, traveling back in time to meet the Boy of Steel. By the 30th century, Superman's heroism had become a matter of legend. It inspired a group of aliens and metahumans to band together as superheroes, defending their time against terrifying cosmic threats.

The core Legionnaires have always been Cosmic Boy, Lightning Boy (sometimes Lightning Lad), and Saturn Girl. Over the decades, they've been joined by other key characters who've become iconic DC superheroes in their own right; the most notable of these is undoubtedly Brainiac 5, a super-intelligent alien.

RELATED: Rumor Patrol: Warner Bros. Wants Legion of Super-Heroes' Movie

DC has rebooted the Legionnaires' timeline twice, and there are promising signs a new version is soon to return in the comics. Every reboot has kept certain consistent themes and ideas, most notably the distinctive Legion flight ring.

How Supergirl has Introduced the Legion

It's long been clear that the Legion exist in Supergirl's timeline. Season 1 saw Kara visit her cousin's Fortress of Solitude, and eagle-eyed viewers spotted a Legion flight ring. It seems that, at some point or another, the Legion will travel back in time and team up with Superman.

Season 2 took two major steps towards introducing the Legion. It introduced two characters associated with the Legion - Lyra Strayd, a refugee from the planet Starhaven, and the Daxamite Mon-El. Both are Legionnaires in the comics, and fans immediately deduced that Supergirl would soon introduce the Legion. No surprise, the season ended with Mon-El forced to leave Earth, and his ship was caught in a temporal anomaly. It wasn't hard to guess that he'd be sent to the 30th century, and become a member of the Legion of Super-Heroes.

The current season has finally introduced the Legion, and revealed that Mon-El was stranded in the 30th century for years. He told others of Supergirl's heroism, and inspired the Legion. Mon-El eventually concluded that he would never return to his own time, and let Kara go. He actually fell in love again, with Saturn Girl. A Legion adventure saw the team blasted back in time, and they were forced to go into cryogenic suspension. They emerged in the present day, and are proving to be valuable allies for Supergirl.

This Could Set Up A Tremendous New Series

The Legion of Super-Heroes have always been popular, and Supergirl is clearly setting the team up for an important role going forward. So far, viewers have only been introduced to three members of the Legion: Mon-El, Saturn Girl, and Brainiac 5. Other Legionnaires are still contained within cryogenic pods, and could easily be awoken as the series continues.

RELATED: Supergirl: Mon-El Gets His Comics Costume in CW Midseason Trailer

Crucially, though, Supergirl has also set up an intriguing plot for the Legion. It seems that the 30th century is facing a terrible threat, one known only as "the Blight." That may be a deliberate reference to a monstrous creature from the Justice League Dark comics, a gestalt entity formed from all humanity's dark emotions. Or it may instead be a completely new creature. Whatever the case may be, Supergirl has clearly set up a major new enemy in the Legion's own time-period. It seems the entire universe, is threatened by the Blight, and only the Legion can stop it.

It's likely that Supergirl Season 3 will introduce more Legionnaires before this arc comes to an end. It's also set up the potential story for a Legion of Super-Heroes series.

The Problem with this Approach

Here's the catch: The CW may have a reputation for delivering DC superhero series, but it doesn't want to have too many. The network recently took a staggered approach with Supergirl and Legends of Tomorrow, and the two series are sharing the same timeslot. The most important reason for this approach was production issues with Supergirl, but CW president Mark Pedowitz told TVLine that they also didn't want five superhero shows on at the same time. With Black Lightning, Supergirl, Legends of Tomorrow, The Flash, and Arrow, he fears it's possible to "overload" audiences on superheroes.

That suggests The CW is currently at something close to "peak superhero," in which case fans now have no choice but to trade off one series against another. If the network is going to approve a new series, unfortunately it's also likely to cancel an old one.

Legends of Tomorrow is Vulnerable

Sadly, the most vulnerable series of all is Legends of Tomorrow. Season 3 has seen the series finally find the right balance between deep, emotional moments and geeky humor. The plots have been consistently top-rate, including tremendous riffs on cult classics such as E.T., and the quality of the show's character-work has been strong. Unfortunately, it seems most viewers made up their minds based on the weaker first season, and aren't coming back.

Season 1 launched to strong ratings, but viewers gradually slipped away. Even a Season 2 premiere featuring Arrow star Stephen Amell failed to draw fans back, and so far Season 3's figures are on a par with Season 2. The "Crisis on Earth X" crossover bolstered the show's performance, but it was a short-term benefit. Even a powerful, emotional goodbye to one of the Legends didn't lead viewers to stick around once the crossover was finished.

RELATED: Legends of Tomorrow Episode Will Feature Young Barack Obama

Meanwhile, the lessons learned by Legends of Tomorrow could easily inform Legion of Super-Heroes, meaning the first season of Legion was much stronger. Both are essentially superhero sci-fi shows, and both could explore the same mood and style. The difference is that Legends would be a fresh series, spinning out of the ever-popular Supergirl. Where viewers have already made up their minds about Legends, they'd hopefully be willing to give Legion a shot.

_

Conceptually, the Legion of Super-Heroes are far too thrilling an idea to be restricted to just one arc in Supergirl. It's already possible to see the characters' tremendous potential, and Supergirl has even set up a first-season arc for them. Unfortunately though, if the Legion is to take advantage of this opportunity, The CW will need to make some very difficult decisions. And that might mean the story of the Legends should come to an end.

MORE: Supergirl: The Great Disaster Could Now Be Part of The Arrowverse