for what it's worth...
1) I'd like to think there was some sort of conclusion to the Angua/Carrot arc. Whatever else TP writes I'd hope that would be included. It is implied or stated in various places that humans and werewolves CAN interbreed but the results can be rather disconcerting, and that wolves don't accept human-shape werewolves OR wolf-shape ones, so I suspect that this will be a bitter-sweet parting of some kind. I suspect that with Wolfgang dead, Angua appears to be the de-facto heir to the title and if the Baroness is anything to go by, werewolves could be matriarchal if the occasion required it. Vetinari and Lady Margolotta could well have a hand in this.
2) I think the whole Sally/Angua lesbian thing is making something out of nothing. Likewise Colon and Nobby, they are just two old sweats who go back a long way together. Nobby shows a romantic interest in Tawneee, as well as Verity Pushpram, although the latter is essentially a running gag. Colon is married with at least one child, and appears to have simply lost interest in sex as many men do in late middle age. I don't see any suggestion otherwise.
3) I'd feel that Vimes is about played out as a solo character, after Fifth Elephant, Jingo and Thud. He does very little in Monstrous Regiment. The only real option is that he semi-retires.
4) Colon was made a Captain amid general misgivings, and it didn't work. No-one has suggested Nobby, although he did become a somewhat questionable member of the aristocracy for a while. Cheery is a technical specialist, not part of the command chain and doesn't really show anything to suggest otherwise; she might be made a Captain for administrative reason, to expand the lab services, though.. Detritus is clearly unsuitable and if he appears again, perhaps more could be made of his private life vis-a-vis Ruby and Brick?
5) I like Moist von Lipwig...
6) I felt that Nation disappointed, it's the only Pratchett book I have started and not finished.
7) I don't have high hopes of his SF collaboration, given the way Larry Niven's collaborations with Jerry Pournelle turned out. They sell, because names sell in that field, but to me the two writers are simply too different. The Mote In God's Eye was startlingly new and different, the sequel less so, and the rest I could take or leave and have mostly left.
8) speaking of Niven, the Tiffany Aching books are not my favourites and to me, TP appears to have fallen into the "Known Space" trap with I Shall Wear Midnight.. Niven became increasingly involved in trying to wrap up various threads and characters which were originally completely unrelated ( The Ringworld books have a particularly bad dose of this ). Niven broke out of the trap by starting the "Draco's Tavern" series of shorts which don't form part of the "Known Space" canon. The treatment of the older and younger Esk Smith and Wee Mad Arthur in particular seemed contrived; the "time paradox" of older and younger versions of the same character was done much better in Night Watch, I thought.
9) I'm unsure about more on the subject of Nutt and Glenda. That seems a bit squicky to me.. I know that TP follows Tolkien's lead in stating that the Orcs are bred from men in some way, but even so...
10) likewise Jools and Trevor Likely. The kitchen scenes from Unseen Academicals are quite good but don't really seem to lead anywhere.
11) The Feegles are a good joke but have rather run their course for me, unless TP has something new to say on the subject? The Igors are getting a bit that way, too.
12) Fifth Elephant and Thud seem to have pretty much covered the subject of dwarves in Ankh-Morpork, too.
13) given the general Ruritanian setting, and the fact that they appear to be on the route of a main clacks line which is hardly likely to go THERE, the implication must be that Borogravia and Slobenia are peripheral to Uberwald in some way. Angua's presence in Monstrous Regiment and her general background suggests that her future is as a diplomat of some sort?
14) what about Sgt Jackrum in this context, as some sort of Kipling-esque "fringe-of-Empire" character? Running a coaching inn, knowing people, acting as a go-between?
just my two-penn-orth...