There's an insane, thoroughly unsourced rumour floating around that the 50th Anniversary special's major guest star, legendary actor John Hurt, is going to be playing... the Doctor.
Apparently, he will not just be cast as a possible future or alternate incarnation of the Doctor, but the real 9th Doctor, an incarnation that was somehow forgotten or misplaced during the insanity of the Time War. But if you're thinking this is all part of an absurdly elaborate "Screw you" to Christopher Eccleston by Steven Moffat, the rumour claims that, as Hurt emerges as the forgotten 9th Doctor, Eccleston becomes the 10th, David Tennant the 11th, and Matt Smith the 12th. Note; there's no clear source for this rumour at present.
John Hurt does seem to be wearing items of clothing similar to the eigth and ninth incarnations of the Doctor as played by Paul McGann and Christpher Eccleston: