My guess, and it is just a guess, is that it will be in line with this season's figures. i.e. a bit lower than last year.
Many of those factors will give it a lift, but I think its overnight ratings will suffer because of season 9's performance. If it's well-liked by those who do watch it, word of mouth will enable the figures to recover somewhat by the time the 28 day figures are in.
I think some of season 9's drop is due to the general decline in conventional TV viewing, but this is far too big a drop to be the full explanation. The majority of the decline this time is due to season 8's dislikable, less fun Doctor and heavier tone overall. For me, the best episodes from series 8 were when the Doctor was enjoying himself and taking charge (e.g.
Mummy on the Orient Express). I think many mainstream viewers were put off by season 8 and didn't bother coming back when season 9 began.
I also think season 9 suffered due to clashes with major sporting events at the start, which would have dampened any word-of-mouth that might otherwise have got going.
The pre-season publicity for season 9 was also poor. Whoever came up with the "Same old, same old" tag line needs to find a job in another industry.
The Davros, season opening two parter was not a good choice for the start of the series. Too much fan-service for the mainstream audience for a series opener. It's a good story, i.e. I liked it

, but it would have served viewing figures better if it had been mid-series.
The later time-slot didn't help.
Being almost all two-part stories didn't help (having a few two parters is good, but they should be special events, reserved for stories that warrant two parts).
The gloomy tone throughout season 9 didn't help.
Less accessible stories, less accessible Doctor, overshadowed by Clara's departure (much as I like Clara, I now think it was a mistake for her to continue this series). All these also didn't help the viewing figures.
No matter how subjectively "good"
The Husbands of River Song turns out to be, its viewing figures will suffer because people who were "burned" by season 9 will think twice about tuning in for it. Traditionally, the viewing figures of episodes that follow a particularly disliked episode, will suffer, regardless of how good it is.
Having said all of that, it's not a major concern for me at this point. All it needs is a sustained run of really entertaining, fun episodes. And
The Husbands of River Song, because it's the Christmas special and likely to be less "heavy" and gloomy, could be the start of that.
Note that none of my comments refer to the quality of the writing, as such. It's about not meeting the expectations of a Saturday night, tea-time family audience