When Legends of Tomorrow's Nate Heywood mentioned contacting Supergirl's DEO and Alex Danvers for help to rescue the abducted Sara Lance, it was just a prime example of one of the Arrowverse's biggest missed opportunities. Bringing up National City's alien-fighting organization and Alex's connection to their lost leader was a glimpse at what could have been one of the Arrowverse's strongest crossovers.
Outside of Supergirl, the Arrowverse shows don't often deal with beings from another planet. The Arrowverse crossover featuring an invasion from the Dominators underused the Girl of Steel and her firsthand knowledge of how to defeat aliens by having Oliver Queen bench her at a critical moment in their final battle. The next time a crossover dealt with an alien was the introduction of The Monitor in "Elseworlds" whose actions ultimately led to the destruction of the multiverse in "Crisis on Infinite Earths". Most of the shows still haven't recovered or properly dealt with the Earth-Prime reset that created the new Arrowverse.
Legends of Tomorrow kicking off its sixth season with Captain Sara Lance being abducted by aliens was the perfect setup for a crossover that could have gone a long way in correcting previous Arrowverse mistakes. Even if COVID restrictions would prevent the full teams from both shows from working together, a successful crossover could have been done with just a couple of core Supergirl characters like Supergirl, Alex, or possibly J'onn J'onzz or Kelly Olsen as Guardian. Instead, the Legends are revealed to have not had any contact with the Supergirl crew since Crisis, to the point they don’t even know Supergirl's DEO was destroyed by, ironically, aliens. Legends could have even utilized an Alex Danvers video chat cameo, which would not have caused any production hardships. Surely, the Legends crew could have gotten some useful information from the Supergirl characters.
Legends and Supergirl had the most potential for a strong crossover, based on how well members of the teams worked together before. Some of the strongest parts of the "Crisis on Earth X" crossover were Ray Palmer's Atom working to rescue Supergirl and the team-up of the Arrowverse's two best original characters: Alex Danvers and Sara Lance. The lack of follow-up on that partnership is a disheartening missed opportunity to reunite two fan-favorites who shared a fun dynamic. A new Supergirl/Legends crossover could have shown a bond of friendship between them much like the one every crossover focused on between Oliver and Barry. Unfortunately, developing strong female friendships is something the Arrowverse has explored as little as possible. The female heroes of the Arrowverse are rarely shown interacting with female friends, even on Legends, where well-rounded female characters abound.
Obviously, the strongest argument for a Supergirl and Legends crossover in Legends’ sixth season was that Supergirl’s mostly alien team would have been the perfect backup to assist the Legends with their alien predicament. While the two quirky groups should have gotten the chance to cross over long before Supergirl’s final season, COVID restrictions made it unlikely Legends’ current season could do more than a mini-crossover. In the end, the Arrowverse decided to do even less and skip a crossover of any kind in favor of a couple of throwaway lines of dialogue. With COVID restrictions still in place and Supergirl’s final season focused inward, Supergirl and the Legends of Tomorrow's dream crossover will likely never occur. While there’s always a chance a Supergirl character could hop aboard the Waverider on a future season’s adventure, it will always be a huge missed opportunity that the Arrowverse didn’t capitalize on their crossover potential.