Spoilers ahead for the penultimate episode of The Flash Season 6, called "Pay the Piper."
The A plot of "Pay the Piper" pitted the good guys of Earth Prime against Godspeed with a reluctant assist from Pied Piper, but the real meat of The Flash on May 5 concerned Iris and Co. stuck in the Mirrorverse. Barry and Cisco were reeling from the news that Iris and Kamilla were replaced by doubles, and they have no idea that Iris and Kamilla are working to free themselves from the other side of the Mirrorverse. With only one episode left in Season 6, I say Iris needs to stay where she is.
Now, I don't say this because I want Team Flash to suffer or Iris to still have to go through her own personal hell, which has ranged from being replaced to watching her husband unknowingly hook up with her replacement to being betrayed. Iris should stay in the Mirrorverse in the Season 6 finale for two reasons: Season 6 is ending early, and there's still a lot of potential to the story in the Mirrorverse.
Thanks to the production shutdowns that impacted the television industry, The Flash has to end its sixth season three episodes short of its original 22-episode order. Episode 19, called "Success Is Assured," was never supposed to end Season 6, so if Iris returned in Episode 19, that would mean no more of Iris in the Mirrorverse when The Flash returns for Season 7.
Sure, fans wouldn't have to spend what could be an especially long hiatus worrying about Iris being trapped on the wrong side of the Mirrorverse if she makes her escape in the finale, but Iris has been stepping up and crushing her own storyline in the second half of Season 6. Her meaty storyline in the Mirrorverse has also taken Barry and Co. on Earth Prime in some fresh new directions, which is important for a show that has been on for as long as The Flash.
An escape from the Mirrorverse in Episode 19 as the Season 6 finale would also mean The Flash probably rushes Iris (and possibly Kamilla and Singh) out of the Mirrorverse, which would be a disappointment after how the show has taken its time with her story. "Success Is Assured" won't be one solid hour of Iris and Co. in the Mirrorverse, after all! In fact, the episode description doesn't mention anything about what Iris is up to. Take a look:
Barry (Grant Gustin) considers a risky plan to save Iris (Candice Patton) from the Mirrorverse. Ralph (Hartley Sawyer) attempts to stop Sue (guest star Natalie Dreyfuss) from making a grave mistake that could destroy her life.
Personally, I'm hoping the episode description doesn't mention anything happening in the Mirrorverse because the story there is too big to be spoiled, even if I am rooting for the story to end with Iris still separated from the rest of Team Flash. Whatever happens, "Success Is Assured" will have to wrap The Flash until Season 7.