I really hope Tom does not return, as much as I liked his role, he's simply too old. Unless he puts on a wig and loses weight of course, they might pull it off then. I don't want that whole "ageing" crap we got when Davidson returned. Also, a pre unearthly child 1st doctor would be very interesting, although I'd feel cheated because only three of the actual actors would return.
I know for a fact Tennant and Mcgann will return (Smith will automaticly be in it), I highly doubt Ecclesten will return, although it'd be nice to see him back. Personaly, the only other fourth doctor I could see returning is Silvester Mcoy, He's not too old yet, it's a good thing he was getting on a bit when he played the role, and he was really getting on the the 1996 film. Plus Mcoy is probably the most enthusiastic actor to return, apart from Collin, but he's never comming back, he's fat and colour blind. If we can't get any of the others, then yes, a pre uneathly child 1st doctor is likely to return.
As for the ice wariors, it'd be really nice to see them back, but I'd have liked to see them back earlyer. It also wouldn't feel right to have a 50th anniversary special without daleks, but they'd have apeard recently, and we sure as hell are having no more of those god awfull Moffat cameo momments. Maybe the Master would be good, but he also appeared in the five doctors. Maybe him working with the ice warriors, just to get the Pertwee nostalgia going which made the show through the early seventies. I'd like to see Simms back, maybe with a little beard this time. But then again, he may need to regenerate, I presume Moffat won't dig skull master again (which I loved though).
Even though Moffat has made a really bad job with series 6, I have high hopes for this 50th anniversary special as he is
capable of making some really good episodes. Even though it's a long shot, I'd really like RTD to help Moffat with the script. The two together are just plain great, but Moffat on his own usualy sucks donkey balls.