I am not sure about this, her leaving seemed pretty conclusive. That said, it does seem to be a habit of bringing back companions, albeit briefly (Sarah Jane Smith, Captain Jack Harkness), why don't they go the whole hog and bring some more from the classic series.
School teacher, travelled with the First Doctor
Highlander from the Eighteenth Century. Travelled with the Second Doctor
Drawbacks: Had memories of his travels with the Doctor wiped by the Time Lords, so introductions would be rather interesting
Third Doctor's assistant when he was UNIT's Scientific Advisor whilst exiled on Earth
UNIT's Medical Officer. Travelled briefly with the Fourth Doctor and Sarah
Drawbacks: Ian Marter, who played him, has been dead for about twenty years and getting another actor might not be ideal.
Nyssa of Traken
Scientist, travelled with the Fifth Doctor (although joined just at the end of the Fourth Doctor's adventures)
Travelled with the Sixth Doctor. (Joined at the end of the Fifth Doctor's adventures)
Drawbacks: The Doctor having to explain why she was dumped on a Planet in the Twenty-Third Century and why he was nasty to her beforehand.
Travelled with the Seventh Doctor
And to those who are fans, admit it. You are rather glad Adric can't return, aren't you! ;)