It also mentions Sarah-Jane, which is a speculation that has no evidence to back it up... way to go there. Anyway here it is:
Sarah, 43, kicks off the fourth series this spring playing mysterious villainess Miss Foster — “an enigmatic and powerful businesswoman” armed with her own sonic screwdriver.
She meets the Time Lord in the first episode next spring during an alien emergency in modern day London.
An insider said: "This series is going to be the best yet. Sarah’s a cracking actress. Sparks fly when Miss Foster meets Donna. She soon causes the Doctor all manner of grief. It’s more than a bit of woman trouble.”
She said: “I’m absolutely thrilled to be in it. It’s a brilliant episode and I’m looking forward to taking on the Time Lord.”
And two of the Doctor’s former assistants will return. Billie Piper and Elisabeth Sladen will replay their roles as Rose Tyler and Sara Jane Smith.