http://www.radiotimes.com/news/2015-11-23/steven-moffat-finally-reveals-why-john-hurt-replaced-christopher-eccleston-in-the-doctor-who-50th-anniversary-special

Moffat looks back at the struggle to make The Day of the Doctor, which aired two years ago tonight.

i don't think anyone seriously thought that Christopher Eccleston would ever even consider having anything to do with Doctor Who again - he's made it pretty clear that HE'S OUT.