If this episode is going to feature multiple Doctors - and in having Tennant join the cast, then it is - then it is a waste and something of a cheat not to include any of the pre-revival Doctors. It is, after all, the 50th anniversary episode, not the 8th anniversary. Paul McGann could quite conceivably play 8 again and I don't think Sly McCoy looks particularly old; he has one of those faces that looked middle-aged in his 30s and now doesn't look a lot older than he did in the 1996 movie. So he could and should have some appearance.
I personally would love to see Tom Baker or Peter Davison appear but I can see merit in the argument that they and Colin look too old to reprise the roles, Time Crash aside. I hope that at the least they'll be invited for voiceover work, or even to play non-Doctor roles.
Of course, as I've said elsewhere, Matt Smith said to Jonathan Ross, when asked to describe this story in one word, 'pictures.' That could mean we'll see pictures of the older Doctors. But I can't help wondering if there'll be some sort of Dorian Gray aspect to the story. Portraits, probably holographic, of the earlier Doctors in the TARDIS, which age, even as the incumbent remains young and vital. Could this be a way of having Tom, Peter and Colin back on board?
Beyond that, anything is a bonus and the other issue is how crowded a storyline is going to get with so many Doctors appearing. I suppose it's possible that the actors playing Hartnell and Troughton for the Adventures in Time and Space movie could also play the First and Second Doctors here too, though I know many of you don't like that idea (with it being a 50th special, I do think the First Doctor needs to be represented somehow).