Well, now we know one previous Doctor that's going to make an appearance during Doctor Who's 50th anniversary. Although William Hartnell, the actor who originated the role in 1963, passed away in 1975, David Bradley has been cast ias the Doctor — Bradley being better known as Argus Filch, the surly Hogwarts custodian from the Harry Potter movies.
Specifically, Bradley's been cast as William Hartnell, for the docudrama exploring the behind-the-scenes creation of Doctor Who at the BBC in the early '60s, titled An Adventure in Time and Space. and not the 50th Anniversary Special itself... that we know of. At any rate, at the moment if the First Doctor does appear in the actual special, it will probably be via judicious editing of footage of Hartnell's past Who performances.
Still, it would be a shame to let such perfect casting go to waste in a single docudrama...
(Photo of Bradley via Ian Gavan/Getty Images Europe)