Reading up on the classic villain
The Zygons have shape-shifting abilities, allowing them to replicate the appearance of another being, but they must keep the subject alive in order to use its body print.
So, anyone want to bet which returning actor won't be portraying the real deal?
Rose Tyler? Billie could play a Zygon impersonating Rose Tyler.
10th doctor? I'm sure fans would lash out at a fake doctor for the 50th anniversary.