Now to think of something that actually counts... The Tenth Doctor? There's three years of his life we didn't see (he became three years older between Waters of Mars and The End of Time), I'm sure they could work him into the next half of this season somehow...
I agree, a multiple-Doctor episode is overdue! Maybe get Ecclestone, Tennant and Smith working together, but it would be nice to have an episode comprising all 11 doctors!
In the 5 Doctors, they got Richard Hurndall to play the 1st Doctor, could do that again for all the ones who have died, although not sure they'd easily find a Jon Pertwee look-a-like/sound-a-like, and both Tom Baker and Colin Baker look nothing like their Doctor characters now...
That'd be really cool if they could pull it off, though I hope that if they did they would come up with a more creative title than "The Eleven Doctors"...