Spoiler warning since this discussion post will likely provoke people to bring up what has happened in recent episodes, as well as potentially correctly predict what will happen in the finale.

So, recent episodes and specials like "The Giggle" and "The Devil's Chord" have been teasing this big villain of the season who is connected to The Toymaker and his child, Maestro. They are presumably part of the same pantheon that these two are in. There are all these mentions and offhanded references to them, and we know that they have a connection to Ruby Sunday.

People have brought up many potential candidates for who it could turn out to be, including Sutekh, Fenric, Omega, The Beast, The Master, The Trickster, Davros, and even the Doctor's own granddaughter, Susan. XD

Of course, it could always be a completely new and original character.

But go on, everyone. Just for fun, speculate who you think it might be, and give your reasons why.