It appears we may not have see the last of The Master in the form of Missy which is a good thing as I really did enjoy Michelle Gomez as The Master. It appears in the recent issue of Doctor Who Magazine (Issue 480 Out Now!) she may have let it slip in an interview that she will be back as Missy in Series 9 of Doctor Who. Here is the quote from Doctor Who Magazine.
“Yes. Yes is my answer. I’ll be back, Can I say that? Am I allowed? If not... well, I guess we'll have to see how she's received..."So is it a tease or did she really let it slip out? We'll have to wait till 2015 to find out if Missy will be matching wits with Peter Capaldi's Doctor again.