Steven Moffat has dropped the ball.
Seems like there will be no returning characters from the Russell T. Davies era - at least for now.
In a recent The Guardian article, new showrunner Steven Moffat talked about the fairytale-like qualities of Doctor Who, saying that:
''For me, Doctor Who literally is a fairy tale. It's not really science fiction. It's not set in space, it's set under your bed. It's at its best when it's related to you, no matter what planet it's set on.''
As for the question of bringing back characters from preceding series, The Guardian reported that
Moffat is reluctant to plunder Who's history without good reason – "The more you back-reference, the more it feels like a sequel and the sequel is never as good as the original" – but that isn't to say that the past is another galaxy. "Old favourites can return, provided you can do something new and exciting with them. There are no past characters coming back in this series, but I imagine that kids would love to see Captain Jack meet the new Doctor."
Personally, I don't think it doesn't mean that Davies's characters will never appear again, but rather that the Moff is looking forward to a fresh start. Plus, though River Song is Moffat's own creation, she was created during Russell T. Davies' reign as showrunner and we already know that she is returning for Series 5.
Though Steven Moffat has written some of Doctor Who's scariest episodes (Blink, Silence in the Libray and Forests of the Dead come to mind) he's also looking forward to lighter, more comical fares
"I want to do more fun. I hadn't done an episode one, which is a very different kind of episode, and I hadn't done a finale either – and that was a hoot."
There's much more to read about Steven Moffat so I invite you to read the complete article here.