As wonderful as the Zygons return in The Day of the Doctor was, their appearance was fundamentally that of a plot-device. In truth any other species could have taken their role in the episode and not much would have needed to be changed. The Zygons as a species are ingenious and they have swiftly become fan favourites despite only appearing in two stories in 51 years. Because of their place in the fans hearts, a story is needed which depicts the Zygons at their full potential. With UNITs next appearance the Zygons will likely appear in cameo form to give some resolution to The Day of the Doctors lingering story thread, but they need a full story to themselves. Imagine visiting their home world, a planet roaming with Skarasen and other deadly monsters. Given Gallifreys return (and Skaros restoration) perhaps a story arc in a later series could see the return of various planets ravaged by the Time War the Sontaran clone planets and the Nestene Consciousness home world (which in turn could lead to another appearance by the Nestene and the Autons?) and this would give way to Zygors return. Deep Breath depicted a tyrannosaurus rampaging through Victorian London. Imagine if that T-Rex was in fact a Skarasen. The CGI capabilities of modern Who would perfectly allow for a believable rendering of the Skarasen to return. Either way, the Zygons next appearance needs to see them as the primary focus.
