Previously, Doctor Who showrunner Steven Moffat released a tease of his own in his Production Notes column for Doctor Who Magazine #426. In a series of vague hints for Series Six, Moffat stated "The startling truth about Idris' new soul -- even the Doctor doesn't believe her!"
So presuming that this isn't some deliberate ruse designed solely to mess with hardcore Whovians, who will Idris turn out to be? According to some baby name websites, Idris is generally a male name of Welsh or Arabic origin meaning "fiery leader" or "prophet," but the Doctor Who character is presumably female due to the casting of actress Suranne Jones. With this in mind, here's my prime list of suspects:
Romanadvoratrelundar, a.k.a. Romana, a.k.a. Fred -- Last seen chronologically on television in the episode "Warriors' Gate," the Doctor's former companion Romana was written out of the series by leaving her behind in the alternate universe E-Space along with K-9 Mark II. In the Doctor Who novels written while the series was off the air between 1989 and 2005, it was revealed that Romana eventually left E-Space and returned home to Gallifrey, where she became Lady President. What happened to Romana during the Time War that destroyed Gallifrey remains unknown.
Jenny -- Last seen in the episode "The Doctor's Daughter," Jenny was a female clone artificially grown from the Doctor's genes. Although the Doctor believes her to be dead after suffering a fatal gunshot, she later returned to life but did not regenerate and was last shown traveling the universe somewhere in a stolen shuttlecraft. It should be noted that Jenny was originally slated to die in the episode, but that changed at the suggestion of none other than Steven Moffat.