I'm still thinking "if Hurt's an alt/lost Doctor, he'll be one who never left Gallifrey, at least until the Time War wrecked the place"...
Makes sense. Further, I suspect that if the Clara rumors are true, then that would be a result of her intervention. Imagine her, spread through the Doctor's history saving his time streams but - ah! - she messed one or two up. Altered the time war a bit, say, so Eight died and regenerated in a different time or place or what have you, and now they've got this alternate Ninth doc who never existed that they have to deal with. Moffat loves his wobbly wobbly universe-imperiling paradoxes, so this naturally threatens to tear reality apart, and there's the premise for the 50th special.