News website, The Music has an interview with Michelle Gomez which confirms her involvement with Series 10.

“Weirdly, this is the sort of crunch week where I get to find out exactly what I’m going to be doing next,” she told The Music. “I can officially say that I will be starting back on Doctor Who soon, so that’s one thing.”

Gomez last appeared in the role as Missy — short for The Mistress, a female incarnation of the Doctor’s long-time arch-enemy, the Master — in episodes one and two of series nine, The Magician’s Apprentice and The Witch’s Familiar.