I'd say Matt Smith, David Tennant and Peter Capaldi are all certain returns. Chris is a bit more nebulous
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Originally Posted by doctor blue box:
“Appearance: Could be a little tricky, being so young in the role means he has the potential to look noticeably different more than any of the others, being that he hadn't already aged as much as them when he first took the role on. Then again he might be fine, I thin he's the hardest to say for.
Availability/willingness: Again he is the hardest one to judge on this subject. He wasn't a fan before he took the role on, and whilst he fulfilled all commitments the role demands during his time, I never really got the sense that he saw it as anything more than 'just another job'. Plus, if his film career takes off, he may well be unavailable.”
TBH, I think Matt holds as much love and affection for the role as Peter and David. When you see the way he interacts with fans, and hear some of the anecdotes about how he got tearey-eyed at the script reading, and passed the torch to Capaldi, its hard to imagine him not pouncing at the opportunity. Even if he's crazy busy with movies, I bet he'd set a day aside for a cameo at the very least.
As for his appearance, while its not something I take into consideration personally (no matter how altered, its the actor returning that counts) remember that he aged to death on Trenzalore, so there's that explaining already nipped in the bud.
Bar health, Davison, Baker and McCoy would definitly be game. They were waiting for the call at the 50th, and when that didn't come they started their own 50th anniversary episode... with Black Jack... and Hookers!
Paul McGann is the most in-demand on the Doctors, but he's just as enthusastic about the character as everyone else is thanks to the Big Finish work he's been doing. Same with Tom. Again, bar health, I certainly think he'd be up for it. He just LOVES being the Doctor.
Chris fretted over going back. He doesn't have as much of an emotional inclination to return as the others do, even if he does hold
some affection for the role. The main gripe with the 50th was that he wanted to see a script before-hand, and since he couldn't do that, he just booked other work. Even David was only asked two weeks prior to filming*. But I wouldn't rule it out. He's just like a Hippogriff. You need to approach him with respect.
*(Feel free to correct me on any of this.)
John Hurt seemed like a one-time deal to me. He came, he Doctor'd, he death scnene'd. One-stop policy. And I'm not too sure he appreciated all focus the role drew. Kinda similar to Eccleston I think.
And while this isn't quite the same thing, I'd honestly accept David Bradley as the first Doctor. It's not a spot on impression or anything, but then neither was Richard Hurndall. Can't really think who would make a good replacement for Troughton

And while asking him to play the role of his father might be insensitive, you can't deny the resemblance is uncanny -
http://imgur.com/3t6cNpW