If there was ever a story in the whole of nu-Who which embodied the term ‘incomplete’ it has to be Ascension of the Cybermen, and in this case it’s in the best possible sense of the word. There are so many threads sown through the penultimate episode of Series 12 that it’s hard to believe, actually that’s not true–it’s hard to trust, that all of these will be satisfactorily tied together in ‘The Timeless Children.’
There are 10 Series of nu-Who (we’ll give Series 11 a pass as it was so determined to be composed of stand-alone episodes) which have embedded this slight skepticism in Doctor Who fans. So often a Moffat two-parter would have a tremendous amount of set-up in the first part only to have a significant tonal (often quite stripped down in the bargain) shift in the second part. The Pandorica Opens and The Big Bang are an excellent example of this.
This doesn’t mean that the back-halves of this type of story mostly don’t work, only that the fan penchant for anticipation makes that kind of disappointment more likely (example: Last of the Time Lords).
It’s all part of the fan experience, that delicious anticipation.
Did we mention that Mr. Chibnall has positively averred that the ‘Ruth’ Doctor will also be explained in the series finale?