Speaking to TV Choice Matt said that an enemy from the show’s original 1963-1989 run might make an appearance in a future episode:
When further pressed Matt clarified that:
Now the biggest and perhaps most favourite returnee beast (the last of the big four left to put in an appearance) would be the Ice Warriors who haven’t hissed menacingly on our screens since 1974’s The Monster of the Peladon.
Although in the monster sweepstakes a strong outsider bet would be the Zygons who haven’t been a major feature in a Doctor Who serial (there have been odd mentions in The Panadorica Opens, a never materialised cameo in Shada and a fleeting appearance in the interactive mini-episode Attack of the Graske) since Terror of the Zygons.
Though let’s not rule out the Yeti, the Raston Warrior Robot, the Nimon (well we’ve already had a descendent of his in The God Complex) and, most importantly, Alpha Centauri (for fun times, make it Centauri times)