I've watched the bonus video of Matt Smith and David Tennant
a few times.
The thought that occurs to me watching it -- someone, some day is going to produce a series of Doctor Who
that stars two Doctors. I'm not talking a single story. I'm talking a run of episodes (8 to 13), with two Doctors. Matt and David clearly have a good rapport in that video, and if there were a run of episodes with the two of them in the TARDIS together it might be nonsensical but it would certainly be a hell of a lot of fun.