We’ve had Daleks, Cybermen, Sontarans, Macra, Silurians and the Master, and thanks to the new out-of-beta Doctor Who: Worlds in Time game there is a possibility that the Zygons could be returning in Series 7.
In an interview on the SFX website new Executive Producer Caroline Skinner is asked in depth about the new browser based MMO title (not to be confused with the PS3/PlayStation Vita game The Eternity Clock). At one point, in a quickly-ignored-but-published section the conversation goes like this:
The Zygons are in it! Is this a hint they’re going to be returning to the show anytime soon?
[Laughs] “Wait and see.”
Interesting that she doesn’t say “no”. Of course, nor does she say yes, but this might – if true – lend credence to earlier reports of a classic-era monster appearing in Doctor Who Series 7.
The Zygons on-screen career as potential masters of Earth has sadly and shamefully been limited to the single Terror of the Zygons serial from September 1975, in which they were defeated by the Fourth Doctor, Sarah, Harry Sullivan and the Brigadier. However these shapeshifters who controlled the Loch Ness Monster have nevertheless been well regarded by fans over the years, popping up in various spin-off books, audios and comic strips.