Easier to say which might be the first to be dropped from the canon
I think A Tale of Two Cities may go, I think someone might write a better novel about the French revolution and render it redundant - it doesn't have the creative originality or unforgettable characters of his other novels. Scrooge, Fagin, "Ghosts of Xmas", ..., are strong, archetypal characters who will never be forgotten as long as literature survives. It would be like ditching Ulysses or Hamlet to ditch these characters. i.e., impossble.