It could be Moffat goofing off, but it's also possible that it's something devious like a relative, not necessarily the obvious. Certainly the amount of attention Myles is getting would indicate she's going to be part of *something*.
It also wouldn't be the first in Who history for a character to have been played by an actor then the same actor goes on to be a different, more major character... Lalla Ward (Romana II) was a character in the previous season, heck even Colin Baker (Six) had been a minor character in one of Five's stories.
This also need not be an old-school thing... Freema Agyeman (Martha Jones) was also in season 2's two part finale, where she is working at Torchwood One in London, and is killed just before the Battle of Canary Wharf begins. It was subsequently subsumed into her character's background when Martha was written that her previous part was a cousin (to explain the "similar face" aspect)
It also wouldn't be the first in Who history for a character to have been played by an actor then the same actor goes on to be a different, more major character... Lalla Ward (Romana II) was a character in the previous season, heck even Colin Baker (Six) had been a minor character in one of Five's stories.
This also need not be an old-school thing... Freema Agyeman (Martha Jones) was also in season 2's two part finale, where she is working at Torchwood One in London, and is killed just before the Battle of Canary Wharf begins. It was subsequently subsumed into her character's background when Martha was written that her previous part was a cousin (to explain the "similar face" aspect)






