I get asked a lot if Persephone is ever going to return as a main character. Short answer, no. Iron Queen is the last book in the Persephone portion of the Daughters of Zeus series.
Slightly longer answer: The ripples the events that occurred in the Persephone trilogy caused are still ongoing. Aphrodite’s trilogy focuses heavily on what comes next for the Pantheon after the boss battle in Iron Queen, and Persephone plays a major role in the Aphrodite’s trilogy. She even narrates a few chapters in Venus Rising. The ending of Venus Rising for sure will have an impact on Persephone’s future, so she will certainly appear in Artemis’s trilogy, possibly even as a POV character somewhere down the line.