So speculating the final order of writers...
10x01 - Steven Moffat
10x02 - Frank Cottrell-Boyce
10x03 - Sarah Dollard
10x04 - Mike Bartlett
10x05 - Jamie Mathieson
10x06 - Toby Whithouse
10x07 - Peter Harness
10x08 - Mark Gatiss
10x09 - Rona Munro
10x10 - Steven Moffat
10x11 - Steven Moffat
10x12 - Steven Moffat
- Moffat doing 1, 10, 11 and 12 is confirmed.
- 2, 3 and 4 are also confirmed.
- Episodes 5 and 9 were filmed in the same block, with Matt Lucas praising Mathieson's script at the time, implying his was either episode 5 or 9. Rona Munro's name cropped up around the same time. Speculatively, Mathieson's episode fits the fifth slot well as his name came up first and (if you reach a bit) he did the fifth episode in the previous series.
- Rona Munro fits the ninth episode, as the decision not to announce names beyond the first four writers could imply they wanted to keep her involvement a secret, and she was then contributing the scripts to episodes 5 or 9 which were next to be filmed. 9 fits better, speculatively, because Moffat tends to put either a new writer or the more recognisable guest writers in a late slot (Richard Curtis, Neil Gaiman, Frank Cottrell-Boyce, Sarah Dollard).
- 6, 7 and 8 are hardest to guess. I put Harness in down the middle for episode 7 - as he did the seventh episodes of the past two series. It also then splits up Gatiss and Whithouse - the two non-Moffat writers who are not exclusive to the Capaldi era, but it's complete guesswork beyond that.
- Catherine Tregenna is the only Series 9 writer not returning. A bit of a shame as whilst I felt the story of her episode was really weak, it had to pander to the Ashildr storyline really heavily and was tactlessly wedged in as a sort-of concluding half to a two-parter she never started, and frankly some of the dialogue was great. Her Torchwood episodes were generally very good with an outstanding one or two amongst them and I'd love to get her back writing for Who again.
- Series 10 has nine writers - at least one more than any series before it. It means we are going to get nine episodes in a row all by different writers (some Moffat co-credits notwithstanding), though I readily await for announcements that prove the above speculated order completely wrong

It's a strong line-up though, and I do hope that Series 10 manages to avoid the trap that Series 7 and to a lesser extent Series 8 fell into which was feeling overly episodic, monster-of-the-week levels of inconsequential.