One would think that Moist has to be somewhere in it. Or, perhaps Adorabelle might be new main character (since she was pretty resourceful, is theoretically still the inheritor of the Gran Trunk, assuming it was sold back to her family after the events of Going Postal). I would imagine that Harry King would be someone involved as well, and it would be nice to see him more narrative time.
One must assume that this takes place on or around the same time as Snuff, which is five or so years after the events of Thud (and probably 3-4 years after Unseen Academicals), since it would be strange for this to be taking place before the Snuff time period.
All I hope is that Pterry somehow doesn't write this in the overwritten Victorian novel style of Snuff and Dodger.