; Date: May 17, 2017
Having read a few articles and such, there's no information about how this will all come about. Ms. Gomez does describe the Master/Missy episode as being like a two-headed monster, which sounds like verification that they'll be in the same episode, the season finale, in the same scenes together.
What we know about the Master's timeline (John Simms returning as The Master, opposite Missy, the female Master and What's the history of The Master? At least 24 bodies .... How did The Master become Missy?) is that The Master has kept living in borrowed bodies. When we first met the Master 40+ years ago, he was in his 12'th incarnation. Then the actor playing that Master died in an unfortunate accident, and they had to cast stand-in's, presenting the Master as some kind of crispy-fried-corpse-like-creature before he took over the body of Nyssa's father (in The Keeper of Traken). He kept that body for some time until the 1995 TV movie, when he briefly took over the body of a San Francisco policeman. All those body changes were not accomplished using regeneration, but by taking over the body of another person. Opposite David Tenant, it wasn't clear whether the Master regenerated into John Simm's body or what, and likewise it was never explained how The Master got ahold of Michelle Gomez's body to become Missy. Did the Master regenerate into those bodies, or were they stolen?
The last we'd seen of John Simm's Master, he was fighting Rassilon and helping to prevent Gallifrey from escaping from the Time War. We don't know how he then became Missy. And that issue is forefront in the question of how Missy will definitively cease such that Michelle Gomez would never return again. For example, if we saw Missy regenerate into another body, that would definitively end Missy's incarnation.
Another aspect of the Master's history is that we never see any definitive ending. Multiple times it seemed the Master was definitively killed, only to return in a few episodes with another cockamamie plan and no mention of how he survived the last disaster.
It's possible that the BBC wants to bury the glaringly obvious possibility of a Female Doctor.
One agenda item Stephen Moffatt had was to establish the precedent that the Doctor could be a woman. Missy was part of that precedent.
Reportedly they've already decided against a female Doctor for the next regeneration. By having Missy fade off into the sunset, maybe the BBC is hoping we'll eventually forget about the possibility of a female Doctor. Maybe. That's a wild-ass idea that popped into my head and has no credence or anything.
Digital Spy tells us that Michelle Gomez didn't realize John Simm's Master was returning to Doctor Who until she got to the read-through. See, she hadn't read the script before the read-through. She arrived at the meeting and it didn't register why John Simm was there, until they started reading the script.
In another article, Digital Spy reports that Michelle Gomez confirmed she's leaving at the end of Season 10. Before the end of the season, Missy will, well: "Something awful happens [in the finale] as well, which is down to Missy just being dreadful. She just can't help herself - there's a moment where she just remembers that she is totally evil. So that is in there... you won't be disappointed!"
Nerdist has a quote "I just don’t want to be anybody else’s Master. Peter was my Doctor, and that dynamic worked so well. You’ve got to know when to stop. You’ve got to quit while you’re ahead. Besides, somebody else can have a go now!" The problem with that reasoning is that previous Master incarnations had appeared opposite multiple Doctor incarnations, and weren't tied to one particular Doctor and/or Show Runner.
At www.reddit.com the question was asked What has been your least favorite part of starring in Doctor Who? The answer, Knowing everything comes to an end. Nothing stays the same. This too shall pass. NEXT! isn't quite an announcement that Missy is leaving Doctor Who.
At www.reddit.com the question was asked There have been rumors that Big Finish want to do a Missy series, regardless of whether its true or not would you be down with starring in them? That no answer was given speaks reams, and we look forward to Big Finish audios of Missy's adventures.
At www.reddit.com the question was asked Your portrayal of The Master is my favorite character in the show. Is there any chance you will get your own spin-off? She answered Missy Who? Make it happen, bitches. Now that’s worth campaigning for. Hurm.. maybe. I really don't think The Master is an appropriate character for a weekly show.