I don't think Doc will be in it. It would be odd to recast him. Possibly a new human character could have Sprocket with him, but I would rather that didn't happen either. (I know, realistically speaking, a man and his dog can't be together forever - but if you don't show them apart, then we can pretend otherwise.) The Fraggle world is so complex I don't think we are going to see everyone and every part of it within a single movie, without it feeling squashed in. It might be necessary to leave some things out for the sake of a better movie - and if the movie is good (as a stand-alone story and not just for fans' nostalgia) then there is more chance that people will want to go and watch the original series as well.