Warning: spoilers ahead for the midseason premiere of Supergirl Season 3, called "Legion of Super-Heroes."
Supergirl has finally returned to the airwaves to pay off on that brutal cliffhanger from back in December when Reign beat the stuffing out of Kara and left her comatose. Reign had nobody to stop her from wreaking havoc in National City, and there was no telling when Kara would wake. Well, the good news out of the midseason premiere is that National City got a big assist from the Legion of Super-Heroes (eventually) and Kara woke up from her coma. The bad news is that the episode ended on a reveal that means Kara is going to need the Legion to stick around for a while. Reign is getting some backup of her own.
"Legion of Super-Heroes" was a rough one for National City as Reign killed everybody who she deemed was denying justice, which included criminals as well as law enforcement. With Supergirl down for the count, the DEO attempted to recruit the Legion of Super-Heroes for help. Mon-El was unwilling for most of the episode, although Saturn Girl was sympathetic and Brainiac 5 was becoming fast friends with Kara inside of her head. Mon-El was finally ready to pitch in when Reign targeted a prison filled with prisoners and guards. The combined forces Mon-El, Saturn Girl, Brainiac 5, J'onn, and Supergirl by the end managed to do enough damage to Reign that she had to retreat and recover.
Reign retreated to the Fortress of Sanctuary, and it was there that she learned that she wasn't alone. The holographic projection who had activated her from Samantha into Reign popped up and told her that there are more like her out there, which we can take to mean that she's not the only Worldkiller on Earth. All she has to do is find the others and presumably activate them to join her in her cause, and that means BIG trouble on the horizon for Kara and Co.
It took all the powers of the mightiest superheroes of National City to temporarily knock one Worldkiller out of the sky. If Reign has more Worldkillers at her disposal... well, Kara is going to need the Legion of Super-Heroes to join her and plan an attack that could reasonably take the baddies out. There may well be more superheroes still frozen for the Legion to wake up in case of emergency, and I think it's safe to say that emergencies of epic proportion are on the way.
Luckily, the Legion's knowledge of the future did reveal in the midseason premiere that Reign will eventually leave National City, although they were short on any details. Even if they had more information, Mon-El might not have been willing to share. As much as he might still care about Kara, he's more concerned with the bigger picture. He didn't even want to enter the fray in "Legion of Super-Heroes" because he didn't want to risk the future. It may take a bunch of Worldkillers to convince him that they need to focus on National City for the time being.
Of course, Saturn Girl and Brainiac 5 could be major allies to Team Supergirl moving forward. Saturn Girl wanted to help well before Mon-El finally agreed, and Brainy seemed positively smitten with Kara by the end of the episode. They could be key in guaranteeing the support of the Legion to Kara and the DEO. We'll have to wait and see.