none of the above.
Can't be Susan or Romana (or Jenny) as they'd have regenerated, not just died and been mysteriously reincarnated elsewhere with no memory of their other self - which also pretty much rules out River, the Rani, any Time Lords...
Silent (or other enemy) secret weapon is most likely out of that list, but I doubt Moffat will just repeat himself like that. Sexy (i.e. the TARDIS) surely wouldn't have died from a fall.
Unintentional side-effect of Oswin's hacking into the Dalek net/TARDIS/etc... maybe.