Yeah, there have been rumours about every past doctor and every past companion at some point - nothing has been confirmed except that Matt Smith will still be in it, though. I hope there are some returns (Tennant would be EPIC), but then again, everyone has past characters that they dislike or wouldn't want to see come back, so if they do bring characters back there are going to be some people who are disappointed.
All the surviving doctor actors in one show would be amazing
Tom Baker, Peter Davison, Colin Baker, Sylvester McCoy, Paul McGann, Christopher Eccleston, David Tennant, Matt Smith. The 8 Doctors.
*head explodes*