*THIS IS PURELY SPECULATION*
I was reading this article (http://www.nme.com/filmandtv/news/steven-moffat-doctor-who-finale-will-change-eve/306741) on the 50th Anniversary for Doctor Who, when a quote from Jenna-Louise Coleman just jumped out at me:
"All I can say is that Clara hasn't just met the Doctor three times before."
What if Clara Oswin Oswald is actually a regeneration of Susan Foreman?
For those of you that don't know, Susan Foreman was the First Doctor's original companion and his granddaughter.
I know this is a stretch, but I hadn't heard this theory and thought I would just throw it out there. Read the article, I would love to hear your feedback in the comment section below!