There has obviously been a lot of speculation about whether there will be a multi-Doctor story for the anniversary (or perhaps before), and if so, which Doctors will be in it.
I have an improbable idea, but it is fun to think about.
What if Matt Smith played a Doctor other than the 11th, as well as playing the 11th? Specifically, he could play the 1st Doctor. The 1st Doctor makes sense because we don't know what he looked like as a young man, and because Matt Smith's 11th Doctor already shares a number of mannerisms and attitudes with William Hartnell's 1st Doctor. With a bit of make-up, Matt Smith could take on the roll of the 1st Doctor, decades (or centuries) before he first appeared in "An Unearthly Child".
I know that there is lots of weird problems with this, continuity and the like, and it is very improbable, but wouldn't it be a fun idea?