I don't know why people seem to think that Missy and Davros can't share the same episode together, or why it has to mean that Davros showing up at the start will be a brief reintroduction so we'll be ready for a bigger role in the finale (although that could very well be the case of course). Some argue that it's overkill, but thinking logically the concept of overkill has never been a stranger to Doctor Who anyway.
I loved seeing the Daleks and Cybermen in the same story at the end of Series 2. I was quite young at the time that aired, had never seen the Classic series, and yet I knew it was something special. The actual exchange between the two was a letdown, but the whole idea of both Daleks and Cybermen in the same story...awesome!
The whole idea of Rose, Martha and Donna in the same story at the end of Series 4...amazing! Chucking in Captain Jack and Sarah Jane was the icing on the cake! Younger viewers love it, and they'd love to see Missy show up, with UNIT, and Kate Stewart, and then...BOOM...there's Davros as well! Icing on the cake. With 90 minutes to the story there's plenty of time and scope for a decent story that will, as Moffat put it, bring Missy back in a way that you'd least expect.
It'd also mean Missy isn't the main threat...which I'd prefer, given we only just had her at the end of Series 8. "All of time and space" we were told at the end of Last Christmas, I want more than more of the same.