1. Don't Bring A Police Box To A Time War
When we saw the War Doctor's TARDIS console room on-screen, it was clearly in the midst of a renovation. We still had a sterile white walls and the "round things" of the pre-War days, but there was obvious signs that the coral theme was creeping in as well. Since this was at the tail-end of the War Doctor's life, it makes sense that the TARDIS would be transitioning to how we saw it under the Ninth Doctor. However, this ramshackle interior seems ill-suited to battling a Time War, which probably involves a lot of timey-wimey antics for which a precise and maneuverable TARDIS is essential. The Doctor also seems to have much more control over the ship now than he did in the classic series, so a big overhaul during the interim would make sense. The idea of a Battle TARDIS has been explored in spin-off media before, but the Type 40 used in the show is evidently intended for travel only. It's not likely the War Doctor (or the BBC) would ditch the iconic Police Box, even in a novel, but he could decide to upgrade the ship's systems to more combat-ready form. Showing a disregard for sentimentality when gutting and souping-up his rickety old TARDIS would be a great way to distinguish the War Doctor from other incarnations and put him at odds with the ship itself. Since the TARDIS is sentient and has a strong connection to the Doctor (as seen in
The Doctor's Wife), how would it react going from a life of exploration and saving worlds to one of combat and destruction? Having the TARDIS clash with "Captain Grumpy" could be a fun subplot to weave into the series, particularly if (as we suspect) the War Doctor is to spend his days sans-companion. At times, it could be used for comic relief and at others it could be a way to show how quick the War Doctor is to anger. If nothing else, it would be a sounding board for the Doctor to say his thoughts aloud and reduce the need for internal monologues of exposition. It may not be enough to spend an entire book focused on, but it'd certainly be a new take on the relationship between the two.