The 70-year-old Australian director revealed as much in an interview with Digital Spy published Monday. Theron received praise for her portrayal of Furiosa in Mad Max: Fury Road this year, but her character may receive little screen time going forward.
"I'm not sure, is the answer," Miller said of the 40-year-old actress' return. "She's not in the Mad Max [sequel] story, but in one of the stories there's an interaction between [Max and Furiosa]. I can't really say more than that because it's still in progress."
The director announced he has two more Mad Max sequels in the works earlier this month, telling Top Gear Warner Bros. is considering two different scripts. He plans to direct a "small film without special effects" before helming the next installment.
Miller wrote and directed all four existing films in the franchise: Mad Max (1979), Mad Max 2: The Road Warrior (1981) and Mad Max Beyond Thunderdome (1985) with Mel Gibson, and Mad Max: Fury Road with Theron and Tom Hardy. The series has earned a combined $534 million.
Theron will instead take over Brad Pitt's role in an adaptation of the Mark Greaney novel The Gray Man. The actress is also scheduled for The Last Face with Javier Bardem and Snow White prequel The Huntsman with Chris Hemsworth.