Along with Twelfth Doctor Peter Capaldi and showrunner Steven Moffat, Michelle Gomez was previously confirmed to be leaving Doctor Who after season 10.
Michelle Gomez hasn’t completely closed the door on a return to Doctor Who just yet.
With that said, Gomez made a wonderful villain on Doctor Who and it could be argued that her character arc was left somewhat unfinished.
Although Missy eventually made the decision to side with The Doctor, the hero himself was completely unaware of this and it would be immensely satisfying for fans to see Gomez’s character return and let The Doctor know that he did indeed help to change her ways.