It will surprise absolutely no one to hear that Missy has confirmed she’s going to be in Doctor Who Series 10. She announced her return in a brief video that’s very… well, blue. No, there’s no swearing!
In the teaser, she says:

“Well, they’ve got me back again. I’m hard at it. Popping on the base coat; it’s a lovely colour of blue I’ve got.”

Yep, Michelle Gomez is back, her last appearance in the show being in 2015’s The Magician’s Apprentice/ The Witch’s Familiar – surrounded by the Daleks, about to be exterminated on Skaro. But she did have a brilliant idea, apparently…

Missy debuted in Deep Breath (2014), albeit briefly, and popped up again throughout Series 8, concluding with Dark Water/ Death in Heaven. It’s no shock that the character would be back for Peter Capaldi’s last series as the Twelfth Doctor – firstly because showrunner, Steven Moffat seems to enjoy writing for her, and secondly… well, because Gomez already confirmed she’d return. Back in October, she announced:

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

The only real surprise is seeing her painting half a TARDIS. I like to think that was Michelle’s idea.
Some are speculating that this is the first of Moffat bringing back his “Greatest Hits,” so what else might we see? Gallifrey? The Paternoster Gang? The crack in time? The shock cameo of Matt Smith’s Eleventh Doctor? If only…
Doctor Who Series 10 begins on 15th April, and we expect the episode(s) to feature Missy to air in late May/early June.