Probably not Bill Nighy. Don't get me wrong - he's a great actor. I just don't think he is the Doctor type. I liked him in the role of the Vincent Van Gogh expert. Maybe he could do a villain at some point. Paul Bettany isn't really that old.
He could be interesting, though I don't really know the scope of his acting skills, so I can't really judge either way. I don't think I saw the kid in anything, but he seems interesting. I guess we'll just have to let them surprise us. There were some rumours about Rory Kinnear, but he denied it all, so it was probably just a false trail so that they would have enough time to ask whomever they want without anyone knowing.