Madison Russell Didn’t Return in Godzilla x Kong: The New Empire, because the film puts a greater emphasis on Kong’s storyline. Madison Russell would be return if the future saquel of Monsterverse installment focuses more on Godzilla.
Madison Russell played a central role in the previous two Monsterverse movies because of her strong connection to Godzilla. However, once it became clear that Godzilla x Kong: The New Empire would focus more on Kong, her presence was no longer essential to the story.
Instead, the film shifts its attention to Jia, played by Kaylee Hottle, who effectively takes over Madison’s role as the central human character. With the movie placing greater emphasis on Kong’s storyline, Jia naturally has a bigger role in The New Empire.
That said, since Madison’s story remains closely tied to Godzilla, there is still a possibility that Millie Bobby Brown could return in a future Monsterverse installment, particularly one that focuses more on the King of the Monsters.