In an interview with the Metro, John Simm was asked if he might play opposite Jodie Whittaker in Doctor Who. He gave an encouraging anwer:
“I’ve no idea but if they asked me I’d absolutely consider it because she’s fantastic. It’s always a pleasure and a joy to go back and do that show and be that guy.”
Showrunner Chris Chibnall has promised all-new monsters and characters for Series 11 which probably means the Master/Mistress won’t make an appearance in any form this series. Chibnall has already stated that Missy will not appear.
It’s also possible Chris Chibnall will chose his own actor if the character appears any future series.