The Flash star Danielle Panabaker says there are currently no plans for another musical crossover, following the show's duet with Supergirl last season. When Supergirl originally debuted, season 1 aired on CBS - placing it apart from The CW's shared DC universe consisting of Arrow, The Flash and Legends of Tomorrow. However, the first season featured a crossover between The Flash and Supergirl that established the latter exists in a different universe within the multiverse (Earth-38 to be exact). Since then, the Girl of Steel's solo series has moved to The CW and become more involved in the network's crossovers.
In fact, Supergirl and The Flash featured a musical crossover last season - following both shows participating in the network's big annual crossover, which adapted the Invasion! comic book storyline. The musical crossover was a hit with fans and critics alike, bringing in Glee alum Darren Criss to play Music Meister, a DC baddie with musical powers. However, the crossover was a massive production, with multiple big music numbers and original songs, and it looks like The Flash and Supergirl are opting not to do a reprisal this season.
The Flash star Danielle Panabaker, who plays Caitlin Snow/Killer Frost, attended DragonCon this past weekend, where she spoke about the show's upcoming season 4. As reported by ComicsBeat, Panabaker seemingly confirmed there will be no musical crossover this season:
"That musical was pretty ambitious. I think by the time we finished the musical, we were pretty exhausted. To the best of my knowledge we’re not going to do that again this year."
This doesn't come as much of a surprise considering there's been no mention of another musical crossover since shortly after the first - The Flash episode, 'Duet' - aired earlier this year and that was largely talk from the showrunners about how they'd potentially be interested in doing another. While The CW announced the first musical crossover during the network's presentation at the 2016 Television Critics Association summer press tour, they made no mention of another such episode during this year's conference. The lack of mention indicated there'd be no musical crossover follow up this season among the Arrowverse shows.
That said, there's always time for the showrunners of either The Flash or Supergirl to decide to do another musical crossover. Even though it would be a great deal of work, as Panabaker indicated in her recent comment, fans would no doubt appreciate the effort. Still, in all of TV history shows rarely do more than one musical episode - unless it's a musical-based series like Glee or Smash. So, while Barry Allen (Grant Gustin) and Kara Danvers (Melissa Benoist) will team up again this fall for the big CW crossover event, it will be sometime before The Flash and Supergirl reunite once more, with feeling.
Supergirl season 3 premieres Monday, October 9 at 8pm on The CW. The Flash season 4 premieres Tuesday, October 10 at 8pm on The CW.