OK, in the tired way of news journalists, this is from "sources close to Terry Pratchett"
There is currently being prepared a new collection of short writings. This will include items left out of Once More With Footnotes as well as works in that collection. The description of the book as a collection of short writings means that anything could be included, quite a lot of the pieces in OMWF are not stories but introductions, prefaces and other articles for books or magazines not otherwise featuring Terry. One item that will definitely be included is the description of Discworld and how he saw it developing that Terry wrote for the proof copy of Sourcery which has never been reprinted in any form, as he had only written five of the books by then this gives a whole new view to what he thought he was creating.
PS. No I'm not about to reveal my source but he is very reliable