I read this one elsewhere. It requires a leap of logic to get it started, but nothing the show couldn't do if it wanted to.

This one posits that Clara is a time lady under the effects of a chameleon arch to make her human, like what Ten did in the third season. The logic leap is the idea that we don't know what happens if someone dies while under a chameleon arch. Maybe they get reborn in the same form? The weird sticking point here is that Clara appears to have been born to actual parents, and how the chameleon arch would accomplish that, I don't know.

But the next part of the theory is that Clara in her time lady form was the Doctor's original wife, Susan's grandmother. She became human in order to hide something, and the Doctor somehow made himself forget her so that it would remain hidden. Now they've met up again, neither knowing who the other is, neither remembering the secret they originally wanted to hide.

It could be some secret from the original days of Gallifrey, or it could be the key to the Time War. But if Clara says the Doctor's true name, that unlocks both of their memories and whatever it was they were trying to hide.

The theory suggests that this is why the TARDIS has a problem with Clara. It knows who she really is, and doesn't want that secret to be revealed.

But that doesn't quite track with the TARDIS showing Clara the book that contained the Doctor's real name in Journey to the Centre of the TARDIS. And the chameleon arch as a means of explaining the apparent resurrection angle strikes me as a leap too far.

However, part of this theory's backing is a twee that Moffat made, jokingly suggesting that Susan's grandmother would be furious to learn that the Doctor got married to River. All the same, I'm not a convert, but I find the idea interesting.