They are not, in and of themselves, remotely spoilery. One of them refers to something from last week's episode, which I won't go into detail about just in case anyone reading this hasn't seen the episode yet. (Although if you haven't seen it yet, what are you waiting for? Go and see it, it's a corker of an episode.)
I should point out that these predictions are not based on any knowledge of the episode, I've not seen it, I've not read a script for it, I've not read any spoilers for it, I'm completely in the dark, and if these predictions turn out not to happen, then, *heh*, I was wrong.
So, with those caveats in mind:
1.) Missy will turn out not to be who we've been led to believe she is. It's not that I have a problem with the revelation at the end of last week's ep (although I know either many people, or at least a vocal minority, do), but I just have a hunch that there's going to be some sort of deeper mystery involved, there need to be a good few plot twists and turns in a season finale anyway, and particularly as this week's ep is an hour long. So, yeah - I suspect that all is not as it appears to be.
2.) Daleks. I don't know why, apart from - as I said - the feeling that there's got to be a little more in the finale, an upping of the ante. What could trounce Cybermen walking down the steps of St. Paul's Cathedral a la the 1960s classic serial The Invasion and the (fairly well known) fact of UNIT's inclusion in the episode? Heck, why not throw the Daleks into the mix too? With so many of the episode's scenes being recorded in full view of the public on location in and around St. Paul's, any possibility of surprise about the Cybermen and UNIT has been blown (hence: use them in the pre-publicity), so they've got to have something up their sleeve to surprise us. They can keep the Daleks in the studio scenes (maybe in the St. Paul's interior?) fairly easily.
Also, a bonus prediction - Seb will turn out to be more important than he appears. There's no way he's just an underling. And if Chris Addison gets some scenes with Capaldi!Doc, that'll be a nice touch too...