He posted something online saying "through the magic of time travel the Master is back."
Exactly how he'll be used in the series is still a matter of speculation. It's also been reported that Matt Smith would be returning.
On top of that some claims to have seen Simms and Gomez on set with another person dressed as a classic erra Master. There's also a photo of Capaldi and the guy who portrayed William Hartnel in Adventures in Space and Time whearing his first Doctor costume. The classic era Cybermen are set to return as well.
I think we're getting a Muli-Doctor story and the first ever Multi-Master story taking place probably on Mondos with the origin of the Cybermen. Obviously new actors would be portraying classic era Master as both Rodger Delgado and Anthony Ainsley have past away. Just like the first three Doctors would have to be recast.
Another possibility is that apperently there's been a lot of backlash about the Mistress and there's a possibility that she may have been lying about who she really is. The theory is that she's going to revieal herself as the Rani. Of course there's no evidence to back this up and it seems like a dumb idea. It would of been cool if she was introduced as the Rani originally but to make her the Master and then suddenly go back on it doesn't make any sense. Why would the Rani want to pretend to be the Master?
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