If you haven't heard John Simm will return as the Master sharing the spotlight with his female iteration Missy and the reason according to Steven Moffat on why he is returning. Here is the very first time we will see the Master pair up with himself to fight off the Doctor which may be IMO the episode that seals Peter Capaldi faith and starts off his next regeneration.
When I first peeped the trailer for a very few seconds I thought I spied the jacket that Peters Doctor was wearing as he was regenerating on John Simms back. I was wrong but for a brief second I had a what if and that would be a cool plot twist that Missy would face the Doctors new persona but this time it would be wearing one of her old faces. Considering Simm's has that outgoing personality he portrays as the Master it would be a fun path to explore. But alas that is not the case and it only will play out in my own imagination.
Doctor Who: Steven Moffat reveals why he brought back John Simm's Master
The Master returns - Doctor Who: Series 10: Trailer