An idea I've had floating around in my head for years, now realized! What if David Tennant really had regenerated in 2008, making Matt Smith the Doctor a year early? Imagine a newly-regenerated Eleven facing down Davros and the Daleks!

Personally I think that if this had happened, there would have been no meta-crisis, meaning no Doctor-Donna. No Doctor-Donna means that the true ending of Tennant's Doctor would be unwritten, thus allowing for the following series of specials to wrap up Donna's time with the Doctor properly before sending him off to Amelia Pond's backyard in 2010.