This is a big speculative article. There are SPOILERS for anyone who doesn’t want to read.
BBC
As the next episode comes closer, the fan theories are going wild. I’ve heard everything from three regenerations in the episode including the Master, Missy, and the Doctor, whether Bill will be coming back, the lot of it all.
The point is, everyone’s guessing at what may happen. A lot of these fan theories are going to be wrong, but one thing keeps popping up. Missy. What’s going to happen when it all comes down to the final scene? Will she stand with the Doctor? Or will she follow her previous regeneration back to the dark side?
As we’ve seen tremendous progress with Missy becoming good, we’ve also seen the Doctor slowly opening himself up to trust her. For any naysayers, it has to be real, since the Master came back. It’s obviously such an issue that he risked time and space to warn her.
“I’m worried about my future.”
So something in Missy must be changing or already changed. She’s already said multiple times that she only wants to be the one to kill the Doctor in World Enough and Time. Which, kind of sounds like the schoolyard version of “I want to be in love with you.”
Even though we quickly saw her go back with her former self, many people are speculating that she’ll decide to stand with the Doctor, as we hear in the teaser.
BBC
Is this the hope of a deluded fangirl, or something that can actually happen?
Even if it does happen, Twissy fans (as they’re known) aren’t counting on a happy ending. Many expect Missy to sacrifice herself and begin to regenerate. This would give Chibnall the Master if he chooses to use the character, and Michelle Gomez off the hook, so to speak.
No matter what happens, it’s a star-crossed love for these Time Lords/Lady. We hope that the theories about Missy turning good stay true, but we also know that some things just can’t change.
Would this change the Master for good? Probably not. We can always use the rule of regeneration that “everything dies” with the person, which could be why Missy doesn’t remember being on the space shuttle. Or, maybe Missy will find what she did not rewarding enough. It won’t be enough to change the character, but the regeneration has a good shot of being swayed.
Is Missy’s good side one of them?
Next: 5 of Missy's Best Moments on Doctor Who (so far)