Since first appearing on Doctor Who during Series Eight, Missy has been the perfect foil for the Twelfth Doctor. That frenemy relationship may continue into the upcoming Tenth Season as well.
Any protagonist can only be as good as their antagonist. No matter how brilliant the Doctor may be, he still needs his enemies and difficult situations to bring out the best within him. A poor enemy can make for quite the mediocre episode of Doctor Who, regardless of the performance of the Doctor.
It is also truly special when there is a great deal of chemistry between the Doctor and those enemies. The Daleks, while a deadly nemesis, have a great deal of respect for the Doctor, as he does towards their abilities. While they may never admit it, and their relationship is truly deadly in nature, the Doctor and the Daleks need one another.
Such is the relationship that the Doctor has with his frenemy Missy. They may oppose each others plans, and may find themselves working on opposite sides, but they also need one another. Their dynamic is truly amazing to watch, and could be enough to make her one of the best recurring characters in the history of Doctor Who.
Fortunately, it appears as though Missy will be back for Series Ten. In a Q&A session (in two parts) that took place with Steven Moffat following a screening of The Husbands of River Song, he took the time to answer questions about what will be coming in Doctor Who. While he was evasive in most of his answers pertaining to Series Ten, Moffat did provide a few answers, including where he stated that Missy would return in the future.
It is certainly great to know that she will be returning at some point. The dynamic between her and the Doctor has been spectacular, and the idea of seeing Missy as one of the first beings that a new companion would be up against is certainly entertaining. Hopefully, if she is indeed slated to return in Series Ten, her appearance will be early on, giving the new companion a bit of a shock.
Missy will be coming back at some point in time, perhaps during Series Ten. Whenever she makes her return, it will certainly be a memorable time.