Thanks to The Day of the Doctor the Zygons have already made a return to our screens, after their magnificent debut (and until that point only appearance) in Terror of the Zygons. It is also thanks to The Day of the Doctor however that the Zygons were sold short – a mere subplot to the bigger issue of the Time War, while at the same time demonstrating why they would have made a formidable enemy.
As shape-shifting menaces go the Zygons are definitely a credible and engaging threat. They are one of the few alien races (with the possible exception of Axos) to demonstrate a genuinely alien technology rather than upgraded earth technology. And of course, who can forget how shocking it is the first time you see a human doppleganger transform into a Zygon – it was a great cliffhanger back in 1975, and there is no reason why the revelation of a Zygon Menace to conclude part 1 of a two part story would not be equally shocking and impressive in a future series of Nu Who.
In some ways I feel less need to make a sell in this particular post. The Zygons’ reputation already preceded them before The Day of the Doctor, and however diminished and diluted their role was, they made such an impressive return that they really need nobody to press their case for them. The only plea I think we the fans need to make is that they are genuinely permitted to be baddies in their own right, without having to be merely a subplot to a bigger story, or (as for the Cybermen in Death in Heaven) simply pawns in someone else’s game.
As for what the story would look like – The Zygons have a very good claim to feature in a whole season arc where one hides in plain sight – and in actual fact you could amuse yourself with a great deal of purposeful misdirection! Across the whole series you could have cameos by seemingly human characters who are, of course, Zygons in disguise – but give them names that have the fans theorising – you could have “The Master of the College” (although I suppose now it would have to be “The Brigadier’s Mistress”) in one episode, but offset that with hints to Professor Chronotis, the Valeyard, the Rani … really your possibilities are endless! The wonderful advantage of the menacing shape shifters is that you can hide them in plain sight, and still achieve a spectacular reveal and cliffhanger.
For that reason alone, I’d encourage Steven Moffat to forget about trying to find Gallifrey and make the Zygons the story arc of Series 9!