Paul Gadzikowski

The Doctor's new traveling companion, Clara Oswin, will debut in the Christmas special. The actress who plays her, Jenna-Louise Coleman, has already appeared this season as a character named Oswin Oswald who (to all appearances) didn't survive the story she appeared in. Head writer Steven Moffat has said of the new companion that she is, I don't recall it verbatim, someone very important in the Doctor's life. I have two very different, but not necessarily mutually exclusive, ideas about who she is.

1. She's someone who gets incarnated throughout space and time so that the Doctor is continuously meeting new people with the same face and manner, and a similar name, everywhere he goes. daibhid_c just voiced this theory on Twitter; so, when I'm posting now in order to establish bragging rights should I turn out to be right, I have to admit he beat me to the internet with this one. But, in doing so, he reminded me that I want to put my other idea out before anyone beats me to it:

2. The theme of Moffat's Doctor Who is, or has turned out to be, how the Doctor finds family now that the Time Lords are gone. River is his wife, as previewed in Moffat's last Davies-era story; in 2011 it developed that Amy and Rory are River's parents. I think Clara/Oswin/Oswald/* is going to turn out to be - in some timey-wimey non-biological way - the Doctor and River's offspring.