Missy is the Rani? I still hope she's not, and I genuinely can't see it happening.
This quote from the director -
The other day when we were doing ADR, adding in the dialogue, the ADR technician slammed the console and said 'oh my God, really? Really?'
It was classic, and I thought yes, it's going to work!
makes it sound like Missy's unveiling is something really surprising. With silly rumours of the Rani's return every time we see a female villain over the past 10 years I can't see it being THAT surprising if it is her. It doesn't seem very Moffat-ish. Unless he's perpetrated some horrible double bluff.
But the second part of Rachel Talalay's quote: "I thought, yes, it's going to work" also suggests to me that the secret of who Missy is, is something really quite unusual, something that we'd never have previously thought of. To simply bring back a cheesy female villain from the 1980s, a villain that hardly any of the viewers will have heard of, seems very underwhelming, especially for the Big Finale Cliffhanger.
I hope, so much, that she isn't the Rani. I really hope the Missy saga concludes with something far more original.