Supergirl signaling Calista Flockhart’s return as Cat Grant before the series ends is not only possible but could be imminent. Invoking Cat's name to cover for Kara's absence at Catco in the season 6 premiere, and announcing plans of a visit from a younger Cat Grant in future episodes, indicates the character's impending return. Plus, Kara's time in the Phantom Zone is going to result in introspection on those who have influenced her, so it would only be fitting to bring back the woman who was one of her greatest mentors, Cat Grant.
Without iconic Cat Grant in season 1, Supergirl might not have become the superhero she is now. So many influential moments in Kara's decision to become Supergirl came courtesy of her "Queen of All Media" boss, role model, and eventual friend, Cat, who was brought to life by Flockhart. While Supergirl was already strong in many respects, it was often Cat Grant who inspired both Kara and Supergirl with the courage they would need to become their best selves. She helped shape not only Supergirl but Kara herself and at times stood by Supergirl when no one else would.
When Supergirl moved from CBS to The CW for season 2, Flockhart chose not to relocate to Vancouver where the show is now filmed. Cat Grant did, however, appear in several episodes to give the character a chance to say farewell. She made a surprise return late in season 2, and amid her return, she finally answered the question of whether or not she knew Kara was Supergirl. With Kara assessing her life in Supergirl season 6, the series can’t end the story without one more visit from Flockhart's Cat Grant, whose departure left a huge void in Kara and Supergirl’s life. Whether the upcoming Midvale prom episodes featuring a young Cat Grant are Phantom Zone hallucinations or genuine flashbacks, the effect will be the same.
Seeing a young Cat's journey will highlight the parallels between her and Kara and reinforce whether Kara was meant to follow the same path. The episode could show that Cat being a journalist was not the only reason Kara idolized her; she saw that Cat didn't have to hide her identity or chose not to, even when pursuing dangerous truths. If it's not a hallucination, then having their lives intersect so early is only a reminder of the importance of Cat Grant in Supergirl's life.
After all, it was Cat who named her Supergirl and eloquently explained why being called Supergirl - and not Superwoman - wasn't an insult. It was Cat Grant who advised the over-eager new superhero to pace herself and not try to do everything at once. It was also Cat Grant who stood by Supergirl when the city turned against her, after she was affected by Red Kryptonite, and reassured the disheartened superhero the city would forgive her. And perhaps most importantly, Cat Grant served as a huge inspiration for Kara Danvers and the career she wanted to pursue.
With Calista Flockhart eyeing a return to TV with the Ally McBeal reboot in development, she may be available to revisit another iconic character. And as Supergirl has shown before, they can pull off surprise returns, such as Chris Woods's return as Mon El in the show's 100th episode. With all the early references to Cat Grant and the appearance of her younger self, it would seem inconceivable to conclude Supergirl’s story without a return from the “Queen of All Media.”